As usual, I revert to my biology background:
"See those front teeth you have? Those are for ripping, shredding, and generally facilitating the consumption of meat. Normal musculature? Oh yeah, that's because we get large amounts of energy from fat, and building materials from protein - in the meat we're supposed to eat. Those digestive enzymes throughout our GI tract? Again, for meat. At least half the systems in our bodies are based around the consumption of meat. We're omnivores - like bears, we'll eat anything (nothing else explains the human fascination with junk food). We're designed to eat meat and vegetable matter. Get the **** over it already!"
Vegetarians are bad enough. When it comes to vegans, I start invoking Darwinism. I have no tolerance for stupid people - and yes, and over-developed sense of sympathy/morality is a form of stupidity. By far the majority of vegetarians and vegans do NOT eat properly and suffer nutritional deficits as a result - which the rest of us get to deal with in terms of health care cost in taxes 50 years down the road if they don't eat themselves out of existence first.
Not agreeing with the treatment of animals is one thing - trying to deny your basic biological existence is quite another.
EDIT: That said, the lunatics on the Atkins diet or those who eat nothing but meat are just as bad.