Going way back in time for this article (2000), but the idea that NASA did tests on sexual positions in zero-G is interesting.

Twenty positions were tested by computer simulation to obtain the best 10, he says. “Two guinea pigs then tested them in real zero-gravity conditions. The results were videotaped but are considered so sensitive that even Nasa was only given a censored version.”
Only four positions were found possible without “mechanical assistance”. The other six needed a special elastic belt and inflatable tunnel, like an open-ended sleeping bag.

Now I wonder whether that elastic belt and inflatable tunnel are standard issue to our astronauts when they fly missions on the shuttle?