Why do things get so uncomfortable when people notice you’re not eating meat? You haven’t started an argument. You haven’t made any moral judgements of your companions.
If you’re asked about your vegetarianism this Christmas, remember that you don’t need reasons not to eat meat. The burden of the argument is on the people who do.
Psychology is at play here. It is reasonably easy for vegetarians to refuse meat without thereby intending censure or blame of meat-eaters. If they are mistaken, then they have inconvenienced themselves only by passing up something yummy and eating something slightly less yummy instead. No moral risk there. If the meat-eaters’ reasons are faulty, on the other hand, they risk immoral action. The meat-eaters feel this moral imbalance and are enraged by it. They would like nothing more than to eat the vegetarian and be done with the challenge altogether. But they can’t, so they must make do with sophistry.
Not needing to give reasons isn’t the same as not having them. But there is rarely much to be gained from giving reasons around Christmas dinner. Things are not balanced. Because meat eaters are normally putting meat in their mouths while they make the argument*. This presents quite an obstacle to their unbiased consideration of those reasons.
Vegetarians don’t struggle with meat eating. The vegetarianism conversation is normally raised by meat-eaters because of the perceived moral challenge of someone who refuses meat. But this is their struggle, not the vegetarians’, because whatever the rights and wrongs of eating meat, it’s always okay to gratefully and politely refuse food.
*Okay, the meat goes in the mouth during pauses in the argument. They aren’t animals, after all.