Living in space presents several problems, but none more prominent than the lack of gravity. Not because it leads to muscle loss, bone decalcification, and incredibly vertigo as those are small things. Rather without gravity there is nothing to counteract momentum so if you were to thrust whatever you were thrusting into would continue floating off in that direction while you either become stationary or worse slide off in the opposite way. Meaning sex is very difficult.
Thankfully NASA has conducted some serious "theoretical" research into the matter. Reportedly involving a very expensive and camera filled pool. There were obvious solutions; straps, harnesses, different positions. But the best solution was an idea taken from dolphins who often involve a third fish whose job it is to bump the others back into positions. Making the next step up from the Mile High Club the Three Dolphins Club.
This also means if you're having trouble selling someone on a threesome tell them it's important...for science!