I've been thinking about representations of sex in movies that really get close to the reality of sex, rather than using sex as a modular treat for the audience. Off the top of my head the movies I've seen recently that got closest to communicating something like my (shockingly limited) experiences of sexuality are The Piano Teacher, The Sacrifice and L'Avventura.
Norman Mailer wrote that Last Tango In Paris fell short of being a real cinematic revolution because it didn't have real and explicit sex. I thought that was nonsense, and I still do. The three movies I've mentioned have nothing close to actual sex, but for me they say so much about sexuality. David Mamet wrote that when we see a movie about, say, a pianist, the filmmakers break faith with us if they show the actor sitting at the keyboard, then shift the camera to show us that the actor is REALLY PLAYING THE PIANO! Movies are about storytelling; we've accepted that the actor is playing a part in a story; to show us what amounts to a stunt betrays faith in our willingness to accept the story on story terms. We know it's a movie; we know it uses artifice to get at truth. Trying to make it more real by reducing the artifice is misguided.
It's much the same with portraying sex. Experientially sex isn't about the biomechanics of what's going on; it's about what's being experienced and communicated. That's why I don't list any porno movies as movies that get close to the reality of sexuality; they're like industrial films, and miss the real point.
I also have some thoughts on another kind of erotic cinema, inspired by my viewing of Baba Yaga last night, but I'll save it for later...