A hot topic these days, and getting steamier as humans face long term habitation in space abord the International Space Station. Now that men and women are both fly in space, it's bound to happen sooner or later. For all we know, sex in space may have already have happened; a husband and wife team have flown together on a NASA space shuttle.

As Arthur C. Clarke wrote in one of the RAMA books, microgravity does interesting things to breasts.

Maybe NASA is already performing secrect experiments on shuttle flights, which positions work the best, and so on. Space porn anyone?