Sex scenes are portrayed all sorts of different ways in movies. The one common thread that connects all of them, however, is that they're often awkward for actors to film. Director Joe Carnahan has been releasing clips and BTS material for his new film Stretch (now available on streaming), and a recent video shows Brooklyn Decker and Patrick Wilson turning it on, and off, at will, and what really goes on during the shooting of a sex scene (some NSFW language follows):
The direction given during the scene may not be obvious in the final product, but like anything, the result tends to be stronger when the actors are doing things motivated by their characters and their intentions. I've directed some slightly more-clothed versions, and the quicker you get through it the better. It helps to decide exactly what the actors will be doing the whole time, but the more you build it up, the more anxiety you can create, and the harder it can be to get the result you want.
Here's the trailer for Stretch: