TL;DR: Think of all of those horrible things animals in factory farming have to go through in their lives. Then imagine a pandemic of a superbacteria caused by factory farming (and the science behind how that is bound to happen is way easier to understand than the science of global warming). Then, try to imagine the most insane comments somebody can make about those things. You know, weird ways of trying to deny it is even happening, insane ways of trying to justify it... Well, chances are, some blogger out there has actually said those things you imagined.

UPDATE on 06/06/2022: This article was written pre-COVID, back when neither side tended to take the danger of zoonotic pandemics seriously. My guess is that the vegetarianism debate is going to change significantly after COVID. Anti-vegetarians are probably going to insist that COVID-19 is an artificial virus (which is unlikely at best, and, even so, you cannot claim the same of other pandemics that came from meat, such as HIV) and that eating farmed animals is not dangerous like eating bats not examined by a veterinary is (which is wrong, it is dangerous just in a different way: factory farming might not lead to virus pandemics, but it leads to super-bacteria). My guess is that the "I don't want to take any vitamins." mentality is going to disappear once we have seen how COVID-19 pandemic was made significantly worse by widespread Vitamin D deficiency (which could have been easily prevented just by people taking vitamins).

By "anti-vegetarians", I don't mean people who eat meat, I mean people who spread pseudoscience supporting eating meat (whether or not they actually eat meat). They sound crazy to vegetarians, and I guess to many other people. (If you don't like sarcasm, you can skip to the serious part by clicking here.)

I've been taught my whole life that I could both love animals and eat them, and that eating meat is somehow necessary. Now I'm made uncomfortable by someone pointing me to one of the few obvious facts of ethics, namely, that I can't do it, and to one of the few obvious facts of nutrition, that is, that I don't have to eat meat to be healthy (UPDATE on 23/06/2022: Apparently, one of the common arguments against vegetarianism is that a significant portion of the population is supposedly ill in such a way that vegetarianism would make their health worse. I have no idea how somebody looking at the evidence could think that. A high-carbohydrate diet with little-or-no meat appears to help against heart disease (being lower in saturated fat), it appears to help against type-2-diabetes (either because saturated fats increase insulin resistance or because omega-3-acids decrease it, very likely both), it appears to help against cancer (containing no heme iron) except possibly breast cancer... And even if milk and soy do indeed increase the risk of breast cancer (which I am not sure how well that's supported by evidence), they are easily avoided even on a vegetarian diet. Maybe if you have epilepsy then you should not switch to a high-carbohydrate vegetarian diet (though the evidence even for that is thin). But we cannot pretend that's a significant portion of the population. And let's not forget a vegetarian diet does not really have to high in carbohydrates.). What should I do now? I can't just admit I've been wrong my whole life. No, I will make a blog-post (or a series of them) spreading rivelous misinformation about nutrition, animal psychology, and other things more or less relevant to vegetarianism.
What should I start with? Yeah, I probably need to make myself sound non-authoritarian, and to make vegetarians sound authoritarian.
So, if you happen to be a vegetarian, I have no problem with that. What I do have a problem with are people who think they are more of a person because they don't eat meat and who are vegetarian because they think eating meat is somehow immoral. In other words, I have a problem with people who are thinking about ethics and sometimes choose what they eat based on that. Eating meat is moral, because...
Well, I was told by my parents many times that it was nutritionally necessary. I haven't really looked much into it, but I will nevertheless go with that. So, let's look up some nutritional arguments against vegetarianism on-line. There we have it: the vitamin B12. Allegedly, vegetarians are more likely to suffer from its deficiency, which causes mental illnesses. Of course, I will even bring up some mockery about how vegetarians are too stupid to realize that because they don't have enough vitamin B12 in their brains. Of course, I will forget to mention the fact that about a third of the world population today suffers from vitamin B12 deficiency, not because we are not eating enough meat, but because of the way meat is produced today. Vitamin B12 is produced by bacteria in the intestines of some animals (including humans, but the amount that's produced by bacteria in human intestines is not even remotely enough to keep us healthy), and those bacteria are usually killed by the antibiotics the farmed animals are regularly fed with. The meat you buy in a supermarket (apart from fish) probably doesn't even contain vitamin B12. How come I didn't know that? Well, I probably did, everyone who knows what vitamin B12 supplements are probably knows that, I was probably just being dishonest.
Of course, I have some more arguments, even less sensical to people informed about relevant things. For instance, I can find and link to some quacks claiming that the Inuit people, whose traditional diet is mostly meat, don't get heart disease. That's supposedly because the omega-3 acids prevent heart disease. Thereby, I can claim vegetarians have a higher risk of heart disease. Of course, again, animals raised for meat never see grass to get the omega-3 acids from. Also, if I did even a tiny bit of research on-line, I would have known that all or nearly all studies done about it have found that vegetarians have a slightly lower risk of getting heart-disease compared to the rest of the population, and that it's been known since the early 1930s that the Inuit people don't really have lower risk of getting heart-disease compared to other American Indians. Furthermore, the evidence for the well-known "fact" that omega-3 acids prevent heart-disease in humans is almost non-existant. But, yeah, I am going to take meat-industry propaganda as a reliable source of information about nutrition.
I might even bring up the long-debunked myth that human beings need around a third of their energy-intake to be protein (in reality, it's less than half of that, if not way less than half of that). That might convince some people. Needless to say, for those who are willing to do a minute of research on-line, I might make up some unfalsifiable conspiracy theory to explain why the research doesn't seem to support that myth.
Naturally, I have arguments that extend beyond nutrition. I can claim that eating meat is moral because it is natural. That is, even if human beings don't need to eat meat to be healthy (which is still being debated because people are idiots), some animals, such as wolves or lions, actually have to. Therefore, if it's morally acceptable for them to eat meat, it should be morally acceptable for us too. Of course, by that logic, you can justify quite a lot of things almost nobody would agree are moral. Most of the species of praying mantis have eating an animal of the same species in their instinct. That obviously doesn't justify cannibalism. Rape is also very common in nature, even in the species relatively close to us, such as dolphins.
And, of course, I'll claim that it is natural for human beings to eat meat, even though they don't have to to be healthy. This raises an obvious question: How could the first humans hunt game, before the invention of the tools and before they found out how to cook? But I am too delusional to see that question in the air. And I will claim that eating meat is moral because it supposedly helped our ancestors survive, that it gave us bigger brains. Of course, there is not much evidence of that (there is evidence that cooking increased our brain size, that's not the same as saying that eating meat did that), and even if there is, so what? How is it going to help us now? Eating meat obviously isn't going to make us smarter.
Related to that, I could claim that eating meat is moral because it's, well, considered normal by the society. Of course, not so long ago, slavery was considered normal by the society, yet hardly anybody would argue that it was moral because of that. Oh, but slavery required a government to be profitable, and slaughtering animals for food doesn't! It's the free market that decides what is moral and what isn't, and it decided that eating meat was moral. Well, I mean, I don't really know that. Without the government subsidies, meat would obviously be significantly more expensive to produce. But saying that might convince some people who are a bit more libertarian than the rest of us not to apply their ethical principles to animals.
But let me resort to something else. When I made so many ridiculous claims, and some people still follow my blog, I might claim that animals actually don't feel pain. That's how I can feel comfortable while knowing that my dietary choices require animals to be killed at a rate at which they cannot be ensured to die painlessly. And I will try to prove it using the arguments that, if applied to something else, would read like this: "I am a conspiracy theorist who thinks that the Moon doesn't exist. Why would I need to provide evidence of that? Claim that the Moon exist is an unfalsifiable statement, so the burden of proof is on you who claims that the Moon does exist. Yes, you can see it, but how do you know what it is that you are looking at? For all you know, it could be a hologram projected on a big balloon, right?" I know that all the qualified neuroscientists agree that most of the birds and mammals feel pain, but I am not convinced by that any more than a conspiracy theorist who claims that the Moon doesn't exist is convinced by all the qualified astronomers agreeing that it exists. If I do a bit more research on-line, I can cite Descartes, a 17th-century philosopher who supposedly claimed animals didn't feel pain. Even he probably didn't claim that, he said that only thinking beings could feel "pain". By "pain", he probably didn't mean physical pain, but emotional suffering (since he probably wouldn't have claimed animals didn't have other physical senses such as vision). But misinterpreting all the known science and philosophy is allowed here. The real question is what made Descartes think animals aren't thinking beings. Because they don't talk? Well, for a computer, it takes by orders of magnitude more intelligence to say 2+2=4 than to compute it, and who knows about animals.
Or, equivalently, I can make a virtually psychotic claim that plants feel pain. Likewise, I can discard all the evidence that the nervous system is needed to feel pain. Therefore, eating meat would somehow cause less suffering than eating plants. Of course, the obvious problem is, what do the farmed animals eat? Naturally, they eat plants. They eat countless plants during their lifetime. But, if I deny that, hardly anybody would know how to prove me wrong. Just like, if a creationist claims that he (or some other creationist) had discovered a precambrian rabbit fossil, hardly anyone would know how to prove him wrong, even though everyone knows that he is wrong.
Or I could point out to the inconvenient things that might happen if everyone became a vegetarian tomorrow, blowing them beyond any proportions. You know, if everybody stopped smoking tomorrow, some things we are unprepared for might happen, therefore it is immoral to stop smoking.
Or, equivalently, I could make up some lifeboat scenarios and claim that it would be better to eat meat in them than to eat plants. You know, by pointing to the stories of the people ignorant of survival skills who were poisoned by plants and, of course, ignoring the stories of the people ignorant of the survival skills who were poisoned, by, for instance, eating a dog's liver. Or the relatively well known fact that the Great Chinese Famine was made worse by people trying to feed themselves by hunting birds and therefore increasing the number of pests that would otherwise be eaten by those birds.
But, you know, actually, I do feel bad about eating meat. But I think I have a better solution to that than going vegetarian. Perhaps we could legislate some laws that will ensure the animals on the factory farms are treated humanely. Or perhaps even legislate some economically and ecologically completely unworkable solution involving hunting (as if there were even remotely enough game meat in the world to feed the population with it). And why are the animals treated so badly? Oh, it's the government, of course the animals would be treated better if the government didn't subsidize the meat industry and made the animals be killed at a rate at which we cannot ensure they die painlessly. And if the government didn't mandate testing drugs on animals, which has very little scientific value and almost always fails to predict the most important adverse effects in humans, but makes countless animals suffer. Animals need to be protected from the government, not the government to protect them, but they have no voice. But, no, I have a political agenda, I want the expansion of government, and I will use the supposed protection of animals from capitalism as a justification. And that, to be honest, is what this is all about.

I hope you now see why vegetarian bloggers don't respond more rationally to the arguments made by their opposition. Because, if our opposition doesn't write anything more sensical than this parody is, how should we reply? The fact that I happen to be a vegetarian doesn't mean I don't have other interests, such as third-way politics, linguistics and computer science. I don't have much time and interest figuring out what is the best response to those nonsensical arguments, any more than I, just because I am interested in linguistics, want to spend time figuring out what is the best response to the phonosemantic hypotheses and other obviously pseudoscientific fringe theories in linguistics. And I hope you see why vegetarians consider anti-vegetarians to be ignorant bigots: because the only explanation for what they write is that they are either willfully ignorant or are lying, all in an effort to support some (probably political) agenda. I, and most of the vegetarians, don't have a problem with other people eating meat. What we do have a problem with are people who are promoting pseudoscientific claims such as that vitamin B12 deficiency or heart disease can be prevented by eating meat. Because that's called a fraud. I've studied quite a few controversial issues, and the truth has never been so obvious and there has never been so much politicization of science as with vegetarianism.

UPDATE on 1/11/2018:
Serious discussion: Is this blog-post just an example of the straw-man fallacy? Well, I don't think it is. I linked to an example of an anti-vegetarian blog-post on the right-side section of this web-page, and most of the "arguments" brought up here are also brought up there. The author makes the claim that thinking that vegetarianism is the right ethical choice is somehow authoritarian (comparing people who hold that notion to Stalin and Hitler), that eating meat is somehow natural and therefore somehow justified, and that, if everyone suddenly became a vegetarian, some terrible things (though he doesn't exactly specify what he thinks would happen) would happen and that it's therefore morally right to eat meat. Though he didn't bring up those dangerous lies (or, at best, half-truths) about nutrition, it's not hard to find blog-posts using them to make a case against vegetarianism. So, it's clear that a significant number of people active on the Internet believe (or at least claim to believe) those things.
The real question is why. Some people may believe some of those things (though I don't think anyone who knows what the word "meat" means can be ignorant enough to believe those things about ethics) because they are ignorant, but I don't think many people active on the Internet would belong to that category. The reason is something else.
Our society is going through growing pains. It's becoming more and more obvious that the beliefs about ethics it is partly based on are wrong. When somebody suggests a system of ethics that's not based on a religious or a political ideology, but on at least some factual content, people respond with some weird defense mechanisms. Those defense mechanisms are not even rationalisations of one's own behaviour, they are inventing the reasons why not to listen to the arguments made by the other side. Those reasons are usually in the form of ascribing bad attributes to the character of the person making those arguments. It's quite a risky thing to do: if you happen to be right, you just save yourself some time, if you happen to be wrong, you lose all your credibility, if not even the chance to ever be right about the things that are being discussed. It's incredibly hard to fight that.
What contributes the most to that behaviour? Well, I think it has to do with the fact that many people have had their ears filled with the nonsense called Marxism. Marxism, just like the vegetarian philosophy, tends to toss around the word "exploitation". Of course, the vegetarian philosophy and Marxism use this word very differently. In Marxism, this means "taking an unfair advantage of somebody", whichever way "unfair" is defined. You know, people who own factories are "exploiting" their workers, despite the fact that factory workers are (usually) not working there against their will. According to Marxism, "wage slavery" is a new form of slavery, despite the fact that there is, unlike in real slavery, nobody forcing people to work for those wages against their will. They can, unlike the actual slaves, just walk somewhere else. One of the best responses I've heard to somebody who is using the word exploit that way is: "Here is a guy who happens to have a lot of money. Be careful, so that he doesn't give you some in an exchange for some small service, because that's exploitation!". In vegetarian philosophy, "exploit" means, well, killing somebody against their will for your own gain (usually very little gain). Killing sentient beings, for food, clothing or simply for fun, is, unlike the "wage slavery", actually quite comparable to slavery. But people who have had their ears filled with Marxism may not realize that. They either believe in Marxism and therefore believe it's the government's (and not theirs) job to fix the injustices, or they see that Marxism doesn't work when it's applied and rationalize it as exploitation (whichever sense that word is being used) not actually being immoral. Or, worse yet, people end up believing that, since human beings are (according to Marxism) being exploited in sweatshops, and human beings are somehow more worth than animals are, it's silly to care about animals when the immediate goal should be to eliminate the sweatshops (and, as everyone outside of that cult can see, they are doing more harm than good by boycotting the sweatshops). Again, it's incredibly hard to fight that.
Related to that, there are quite a few people, especially the younger generation, who want to replace a relatively well-functioning society with some vaguely imagined socialist utopia. So, when somebody brings up vegetarianism, they quickly connect it to that (as if there was anything more utopian and authoritarian^[1] than the standard "I want to eat meat, but I want the government to ensure the animals are treated humanely." maxim). They end up thinking there is no difference between wanting to replace an obviously terribly functioning part of our society, such as the production of meat, with something less destructive, and wanting to replace our well-functioning economy with something that doesn't even work on paper (because of the economic calculation problem) and has had devastating results every time it's been tried. Once again, it's incredibly hard to fight that, especially if you don't happen to be aware of that.
What also makes it unexpectedly hard to argue for vegetarianism is the fact that a significant number of the anti-vegetarians think that the science is on their side (the same is true for Marxists). Probably not most of them, but still a significant number. Most of the creationists, Flat-Earthers and climate-change deniers know that science and evidence are not on their side, they just think that those things have not been necessarily proven, and that their incoherent hypotheses are somehow simpler explanations. To convince them they are wrong, all you need to do is to convince them to trust science (and that's a relatively easy thing to do). With the meat-eaters who are convinced that the science is on their side, you need to do quite a bit more (explain who is to be trusted about nutrition, explain what kind of evidence counts here, explain why what they have been taught at school may not be true...).
And, let's not forget, there is indeed a load of bad animal rights activism out there. There are a lot of people out there who argue for veganism, rather than vegetarianism. Veganism is based on the idea that owning sentient beings itself is immoral and psychopathic, no matter what we think the consequences would be (just like having human slaves would be wrong no matter what we thought the consequences would be). Vegetarianism is based on the idea that it's immoral to support an industry that relies on causing suffering and death of sentient beings to work. Though I respect the philosophical position of veganism, it's quite obvious that what you do by arguing for veganism is making many more people think you are crazy than the number of people you make reconsider their dietary choices (Most of the people today won't accept either "Socialism is slavery, and it is immoral no matter what we think its consequences would be." as a rational argument when applied to humans, yet alone if it's being applied to animals.).
So, I hope you can put yourself in my shoes now and look at the things more objectively.

[1] So, some people might ask what do I mean by "utopian and authoritarian". It's basically when you think that a problem that's inherent in the system can be fixed just by passing a few laws. An example would be a law that says that websites mustn't track their users, which includes collecting the information about the device they are using. In a perfect world, in which the devices don't have their flaws, or their flaws are easily detectible by a JavaScript program running on them, that law would make sense. However, we don't live in such a world. Web-browsers on various devices used to browse the Internet are incompatible with each other, and those incompatibilities are often not detectable by a client-side JavaScript program embedded in a webpage, it often just continues operating having no way of knowing that it's not producing the desired results. It's sometimes hard to make a website work on some device or in some particular browser even if you correctly identify the device and the browser the user is using. So, let's suppose such a law has been passed (It's actually quite a bit ironic that the laws against the non-consenting use of cookies are often being passed, yet the technologies that actually make it possible for companies to spy on their users, such as Apple iCloud, are legal almost everywhere). What would happen? Thousands of legal and useful websites will cease to work properly on various devices. On the other hand, the illegal websites that want to spy on their users will mostly continue to do that. Now, I hope we agree that the Internet, with all its flaws, does more good than harm. The same, however, can't be said about the meat industry. With the Internet, the incompatibility is the necessary by-product of inovation, of there being new and different (and sometimes significantly better) devices browsing the Internet. Animal suffering is the necessary by-product of the meat industry, because the profits are very low (even with the government subsidies), and attempting to treat animals humanely would increase the costs even further. And that's true not because people are evil and want to be bad to animals or because people are incompetent or careless, it's true because of the nature of the industry. And legislating new laws won't solve it.