From an NPR interviewa couple of years ago:

"On sex in space

The astronauts — NASA and the Russians — continually deny the heavy rumor mill that says it's already happened. The astronauts are well trained and not supposed to do that, but, yes, when the public is up there, they're going to do what they normally do on the earth. ...

"... there'd also be ways in which Newton's third law of action and reaction interfere with the normal methods of sex that you might use on earth. I'd just assume it's going to be an adventure, and people will be creative and they'll find new ways to enjoy themselves." Chris Impey Your body is not functioning normally when all your capillaries and your muscles are designed to deal with the tug of gravity and you won't have that. But there'd also be ways in which Newton's third law — of action and reaction — interfere with the normal methods of sex that you might use on Earth. I'd just assume it's going to be an adventure, and people will be creative and they'll find new ways to enjoy themselves."