This probably goes without saying but I'll say it anyway: real sex on screen is bad for aesthetic reasons, because it takes you out of the movie.
Pauline Kael used to make the point that movies are about illusion, which is why when you see a film where an animal is actually hurt, or a film like Raging Bull, where the real Robert De Niro dangerously fattens himself up just to be convincing, then you're not seeing a story but the real life machinery behind it: you're seeing an animal in pain, you're seeing a fat DeNiro and not a fat Jake LaMotta. Same with this Shortbus thing. You're not watching a story; you're watching actors fuck. You're watching exhibitionism at its most painfully and unpleasantly sincere.
This is an old debate, of course, going back some years, as to how much reality a movie can take, or rather, how much reality a movie can stage, which is what people are now asking of reality TV, where the camera doesn't record reality so much as shape it. Andy Warhol was the trailblazer in this regard, and later Norman Mailer, with those feature-length improv home movies of his, one of which led to some real on-camera blood-letting between Mailer and Rip Torn. Although a number of his films are unendurable, John Cassavetes actually figured out a way for improv to work on film. So does Mike Leigh.
Shortbus director John Cameron Mitchell developed his film with his actors, much as Cassavetes and Leigh did with theirs, but it seems Mitchell's actors were actually maybe a little more skilled (and in one case downright acrobatic) at displaying skin than emotions. When his actors speak, you're reminded just what a really, really, really bad couples movie is all about. Imagine the worst Henry Jaglom movie you can think of performed nude and you'd about have it.