Like some of you who have responded, my fantasy is Voyeuristic. Like watching a porn flick, but in real life with me as the only audience. This fantasy went unfulfilled until a few years ago, when CL was still posting casual encounters section.
I stumbled onto an ad featuring a couple who was looking to have a cash pay audience to watch them have sex.
Turned out they were in there late 20's. She a tall blond, athletic build gorgeous girl, he a nerdy but well endowed guy. We went back and forth on what was going to take place and $. I negotiated an hour watching them have sex followed by some "alone time" (boyfriend left the room) for me and the girl (everything except FS was agreed). I subsequently set up a few other dates alone with her, which were epic. Something about having sex with someone's else's girl and them knowing about it that is a turn-on...
It was an awesome experience that I would love to repeat.