Why do some vegetarians claim moral superiority over meat eaters?

A while ago on Facebook, my online equivalent of the officewater cooler, someone asked this question (I've probably changed the wording):

"When meat eaters have dinner parties, they invariably include a vegetarian dish for vegetarian guests, so why, when meat eaters are invited to dinner parties in the homes of vegetarians, don't vegetarians prepare meat dishes for their meat eating guests?"

It sparked a small and, to me, amusing furore. The vegetarians with no sense of humour over their chosen diet were clearly unamused. Cooking meat, said one, makes your house stink. Clearly she'd never boiled cabbage.

Sermons were posted about the "suffering" caused to animals. Even people like me who try to grow their own meat, ensuring the animals lead a happy, warm, and comfortable life before their quick and humane demise, were not excused.

Eating animals, according to this school of environmentalism, is unsustainable.

Funny, I thought it was humans who are largely contributing to climate change. If it is the farming of animals, then perhaps dairying is the culprit, in which case, we are not eating the cows, but what they produce, and even the purist vegetarians will be consuming tiny fragments of their product in just about everything they purchase, despite their best attempts to go "dairy free". [See Claire Browning's argument from last year here].

(Yes, I know we eat the bobbies later, but let's not digress into an argument over what would happen to this country's economy if we dumped dairying overnight.)

So what about the suffering? The woman posing this argument went on to draw a parallel with animals in zoos "suffering every day". She'd just made a film around horses, so I was tempted to ask if they, too suffered from being trained and ridden every day. Where do we draw the line with animals? Dogs kill rabbits. Cats rape each other, and kill mice and birds. Animals eat animals.

I'm certainly not advocating, nor condoning cruelty to animals, and actually, I could easily turn vegetarian. I'm not that hooked on eating meat. A very good friend eats no meat or fish, hasn't done for decades, but says he'd never dream of being evangelical about his diet. It's his business alone.

He doesn't eat them because he loves them, and because he does a lot of diving, he sees fish as nice critters too. But when I go away he's happy to stay at my place, feed my pigs, chooks and livestock, knowing they're being fattened for slaughter.

So I can't help wondering, these vegos who claim it would make them sick to cook meat for others, as if they would be committing a mortal sin; who pontificate that none of us should eat meat if we wish to save Planet Earth, but back up none of these opinions with empirical evidence, aren't they just Holy Rollers in disguise?