Sex can be hard in real life, but it’s only harder in film, because sex was not designed for a third party to participate. Bodies are twisted with less athleticism than corporeal necessity. Passionate necking looks boring at worst and vampiric at best. Put a light in the wrong place, and passion becomes pornography.
And finally, no one has ever groped a boob and looked cool in the act.
On the film criticism series Frame By Frame, highlighted by AV Club, Matthew Patrick breaks down how Hollywood deals with such issues. Spoiler alert: He introduces Laura Mulvey’s seminal theory of the male gaze, explaining how body parts are pieced together in a highlight reel of testosterone-fueled staring, while walking through the fundamental mechanics of implying a lot while generally showing very little. You’ll finish the piece giggling and probably feeling a lot like Ridley Scott, who had this lovely explanation on why his movies don’t feature sex: "Sex is boring unless you're doing it."
The only side-note I’d mention to Patrick’s fast-paced primer is that he implies that reaction shots—you know, those close up moan faces—are somehow unique or special to sex scenes. Sure, they’re an essential part, but they’re an essential part of all scenes of all movies. Watch any film or TV show, and you’ll see the camera often cuts to the person who isn’t speaking during dialog, or captures someone’s face crumple as something horrible happens on screen. In this sense, faces are an empathetic gateway for the audience—during sexy times, and not.
But that cut to some painstakingly backlit beads of sweat on cleavage? Yeah, Mulvey really called that.