I don't remember where I read it, but someone asked one of the lady astronauts who was on Mir for 9-12 months (or something) whether or not she ever had sex with any of the others on the station. In the interview, she sorta suggested she did.
Frankly, let's be honest: there are few things in life that you can do that are truly unique and interesting; someone else has probably done it before. For that matter, I can think of few more interesting places to screw than in space. It would be awesome no matter how cumbersome or bizarre.
It. Is. In. SPACE!!!
I would not pass up the opportunity. I would hope that, given the very few people who are given the privilege to even be in space at all, that at least one of them would not have passed on the chance either.
What no one is asking is this: who was the first astronaut to masturbate in space? No one may ever know, except that person who was actually first. It would make an interesting story, maybe. "Well, the other two guys where making history walking on the moon, and someone had to stay in the capsule and stuff, so I just took that 40 minutes to beat my walrus....ON THE MOON!"
It would be the very quintissential definition of epic.