What about that plane that you can go on that simulates weightlessness. I think it's only 30 seconds at a time, but hell, who am I kidding, that's plenty of time.
apparently, it's been done
This is one of the few pictures movies ever to be nominated for a Nebula Award. It's also the first pictures movie ever to be actually filmed in zero-G. According to Space.com:
The scene was filmed by flying an airplane to an altitude of 11,000 feet. The plane, containing performers Sylvia Saint and Nick Lang, then went into a steep dive, creating the momentary illusion of weightlessness.
Insiders described the filming process as particularly messy from a technical and logistical standpoint.
Budgeting constraints allowed Saint and Lang, who portray astronauts, only one shot at a perfect zero-G take, leaving the actors with only a narrow 20-second window of time in which to launch themselves toward one another and complete the scene.
http://io9.com/5686804/12-greatest-zero+gravity-sex-scenes-of-all-time-[nsfw]
