I've long since stopped being surprised at the hostility with which people respond to vegetarians. Meat-eaters usually treat vegetarians and vegetarianism with anger or with mocking. I can't help but see this as defensiveness. People don't come to the question of whether it is right or wrong to eat animals objectively; most people are heavy meat-eaters before they consider that it may be ethically wrong. Therefore arguments tend toward justification of desired behavior, not toward legitimate debate.
That said, here are two of the more stupid arguments I've heard in favor of animal consumption. I assure you these are not Straw Man arguments; I have seen people use these types of arguments.
It is instinct for humans to eat meat.
This is simply not the case--unless you are out in the woods, catching and killing animals with your bare hands, and eating their flesh.
What we eat is almost entirely social; we eat the foods that society provides for us as acceptable options. Most people don't kill what they eat; they simply purchase it. Society tells us that meat is an acceptable option, so we eat it. Indeed, many people only eat meat that has been processed to the point that it no longer resembles the animal it came from; for many, there is no connection whatsoever between a piece of meat and an animal.
You don't believe that animal consumption for humans is social and not instinctual? Consider these two related points. In American society, people primarily eat birds, cows, and pigs. There are other animals that could be consumed (and to a much lesser extent than those animals are consumed), but primarily that is what we eat. That is partly socialization--these are the animals (that admittedly through convenience) are found acceptable to eat. Furthermore, if animal consumption is "instinct," you better be careful around your pets. Your instinct just might force you to kill and eat your cat or dog! Of course, that's silly. We don't eat our pets; some people are horrified to learn that dogs might be eaten in other cultures. We have made a CHOICE about particular animals. We are ABLE to make a choice about particular animals.
My cat is a prisoner of her biology; she must eat meat to live a healthy, thriving life. Humans are, of course, prisoners of their biology in many ways. One way we are not prisoners is regarding animal consumption; our bodies don't need it, we don't have a natural "instinct" to eat it, and our bodies may be healthier without it.
But people kill plants to eat them! What's so different? You don't care about killing plants, why do you care about killing animals?
It is undeniable that organic beings need to consume the products of other organic beings for survival.
However, if you can't see a moral difference between and animal and a plant, you are really not worth my time.
A chimpanzee has 99.9% of human DNA. Other animals have personalities and desires. Cows, chickens, and pigs have personalities and relationships. It is beyond preposterous to suggest a moral equivalence between a creature with a brain and a tomato.
Addendum: after seeing the "But we eat plants!" argument again, I've realized it's a version of the Slippery Slope Logical Fallacy. Slippery Slope implies we don't have the ability to set clear, logical lines. But we already set lines between humans and animals. We (irrationally) set lines between some animals and others. Is it so illogical to set the line between animals and plants? To me, that seems like the most logical place to put the line.