I tend to think that if somebody says no to a particular food, then there are broadly four categories of reason.
Can't eat it: eating that food will cause an adverse physical reaction. Of course, this is the most serious reason. My brother can't eat peanuts, his throat will swell and he might die.
Won't eat it: people with religious/ethical/broader health reasons go here. I won't eat meat. If I accidently eat it, I will be disturbed and upset (like my muslim stepfather would be if he ate bacon), but I won't suffer adverse physical consequences, I won't need medical attention.
Prefer not to eat it: things you dislike based on taste/texture go here. I prefer not to eat pickles. I wouldn't choose them, and if I accidentally eat one, I won't enjoy it, but it's no biggie. I'll have a drink to take the taste away and move on. The point is, I won't ever eat a pickle out of choice. Not today, not tomorrow, not ever. I just don't like em.
And finally, we have the least serious category:
Not today thank you: a food that I would ordinarily eat, but don't feel like having today. I have eaten it happily in the past, and will eat it happily again, but today I'd rather have one of the other options being presented.
Any one of those four reasons is completely valid as far as turning down the meat option at a gathering. But where the non-meat option is in short supply, I think that the folks from the 'not today' crowd should allow those who wouldn't ever choose the meat to go first.