My gay friend asked me to do "stuff" with him and his boyfriend

So yeah, I'm 18 and totally straight and this guy is a good friend of mine and 2 years back I said something along the lines of "If I were gay I'd probably date you" which was meant as a compliment cause he's a great friend and I mean I hear women say that all the time to their gay friends. Well we were talking and he asked me if I remebered saying that, which I didn't at first, then he asked me if I was open, then he asked me if I was willing to try stuff with friends. This grossed me out because obviously I'm not into guys. So the thing is I moved abroad 2 years back but he and another friend who is straight and me, meetup every once in a while (Conventions, movies, go out,...) but next week I'm supposed to go to a convention with them so (my gay friend with his boyfriend, my straight friend with his girlfriend and me) but I don't know how I'll handle this is person, if they like start talking about it or something, how do I deal with this situation?