I'm in a very similar situation. All my friends in my social group are sexual, have sex partners, talk openly about sex, etc. I have gotten used to playing along, laughing in the right spots, nodding in others, but never contributing stories of my own. Mercifully, I haven't had to deal with any friends going off for 'loud sex' just yet. Though if I did, I might instead punch something instead of vomit. I simply find one excuse or another when I feel too uncomfortable, or fear awkward questions I can't answer.