Hmmm....
If I went to watch Tron 2 ( which I didn't ) and had the choice of a 2 minute sex scene, or 2 more minutes of tank/bike action. I'd opt for the tanks and bikes.
As a general rule, as has been cited, sex scenes in SF are rarely there to advance the plot. When I'm watching a Sci Fi movie I want Sci Fi. So please shut the door, fade to black and add in more space ships!