I think the closest I've ever been to having sex in front of someone else is fingering my girlfriend while all our mates and stuff were in the room... We'd be watching TV or a film, all innocent like, with her sitting on my lap, covers over us - with my hand between her legs - that was fun LOL Oh, and I've told this story before, but this other time my GF was really tired after going to a party, but was also very horney, so on the train on the way home she asked me to finger her while she slept - i'm sure she had sweet sweet dreams that night
the closest ive had to anything in public was when i had my first boyfriend. We would hide with a couple that were friends of ours behind the stage and in the stairwell near the stage and sometimes the couple with us would touch one another and we would do the same. I had this pair of capri pants that if I sat in them he could reach his arm up the bottom of the legs and finger me. I let him do that a few times in the stairwell and once in the music room when we were all watching a movie:X