Others are:

How can you do that when people are homeless?

We are harming the meat industry and people will those their jobs.

We are ungrateful by refusing perfectly good food.

Vegetarianism is a mental illness.

We just do it because we think it's cool or trendy.

We are all children and no adults are vegetarian.

We don't have real jobs and we would eat meat if we did.

My grandpa ate meat and lived to be 100.

The "I knew a vegetarian once who did something bad" approach--"I knew a vegetarian who abandoned her children" etc.

The "totally irrelevant health problem somebody once had" approach like "I knew a vegetarian who had herpes, I bet she wouldn't have had it if she wasn't vegetarian."

To Skully: Yes it is silly and compulsive but it's amusing to some of us because we get asked the same things so many times here. If you have a problem with that, you can give answers in other sections. If you don't want to hear about Star Trek, don't come to the Star Trek convention. I have a friend who is into Civil War reenactment; I think it's dumb so I don't go.