As far as on-camera/off camera sex goes, I don't mind it being on camera as long as the reason is more than "the writer wants to titillate the readers".
If it's needed for the plot, then fine. But often you can get the same reader response from a before/build up/about to happen/afterglow view of the couple.
Showing the sex scene must meet the same criteria (for me, anyway) as they-had-sex-when-you-weren't-watching -- how does it advance the plot, and how does it complicate the character's lives? Often those two are the same thing.
Just written an almost-sex scene (the couple were on-camera, they were just about to, and then they were interrupted). This allows me to frustrate my characters, complicate things for them (how do they do it without getting caught?), and set up a re-match in keeping with the plot.