From Space.com:
'Sex is very difficult in zero gravity, apparently, because you have no traction and you keep bumping against the walls," biologist Athena Andreadis of the University of Massachusetts Medical School told SPACE.com in 2011. "Think about it: you have no friction, you have no resistance."To boldly go where... OK. I'll stop.
In spite of the challenges sex in microgravity poses, Woodmansee thinks that the Inspiration Mars journey could motivate other couples to take advantage of less ambitious cosmic destinations, such as low-Earth orbit, should they become commercially available.'