A couple of points.

There are several reasons for being a vegetarian/vegan:

1)Perceived health issues (actual or not)... fairly reasonable.
2)Environmental reasons: supply & demand of meat.... also reasonable in some places.
3)Financial issues: growing a garden is much cheaper than buying meat. If you are a hunter, it's different.
4)Cultural: The world being what it is, some people eat things that others find 'odd'... I would have great difficulty with roasted squirrel brains, even though I don't object morally.
5)BBES...."Big Brown Eyes Syndrome": the usual basis for 'moral' reasons. Some of this is religious (Jainism and others), some just generalized objection to killing other creatures, which varies up & down the scale according to whether creatures are warm-blooded...etc.

Now, to these I ask, when did the religious/moral concerns arise? After all, we are descended from omnivores for at least hundreds of thousands of years, it not millions. I doubt our 500th great-grandparents thought much about it- they were hungry, they ate what they could find. Some eventually did little ceremonies to honor & thank the 'spirits' of their meals... but they ate 'em anyway.
I suspect that as humans' ability to rationalize & imagine stuff like 'souls' developed, so did 'concern' for animals.

I do know that at several folk gatherings I attend each year, the problems of the hosting kitchen staff are more than I would want to take on. To the natural allergies & other medical concerns, we have the whole range of religious & BBES and just plain finicky eaters. I know several people who have such a level of concern that they just bring their own meals.

If I were invited to a gathering that sounded like I would not find anything to eat, and was not allowed to even bring my own, I would simply find a convenient excuse to not attend.