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?