So, I don't describe myself as vegetarian or vegan. My only "rule" that I really try to follow is "don't buy or request others buy animal products for myself."
Yesterday friends I was hanging out with decided on pizza for dinner. I'm pretty severely lactose intolerant and had already had cheese in a salad that was served earlier that day, so my stomach wasn't feeling awesome. Another attendee who's lactose intolerant also wanted an alternative. Burgers was suggested and she was cool with that, and I really didn't want to be a pain in the ass and the grill was already going to be used, so I also agreed to a burger.
To my host's credit, he's awesome at grilling and it was an excellent burger. It did make my stomach a little sour after as I usually get with animal protein (I think this is psychological as I haven't found any scientific studies that show a biochemical difference between animal protein and plant protein digestion).
Struggling with feelings of guilt now though, because it felt like I was breaking the "don't request others to buy animal foods for me" part of my ruleset. The meat was already existing in the household and not purchased for me, but realistically, one more will be purchased now in the future than would have been had I not had it. Ah crap.
The only way to ensure no repeats of this kind of situation without being a huge pain in the ass guest is probably to start bringing my own backup foods to events, but that also carries the potential of insulting hosts. Then again my friends are pretty relaxed, accommodating, and open-minded (they have to be in order to enjoy my company and vice versa lol) so maybe I don't need to worry about this kind of thing causing offense at all.
I guess I mostly wrote all this out as a way to augment introspection. Maybe it'll help others who have had similar experiences not feel alone, I hope.
Guess I've got more thinking to do.