The sex scenes are simulated in The Dreamers. It got its NC-17 rating for the nudity.
I'm shocked no one's mentioned the Japanese film In the Realm of the Senses. Probably one of the hottest art-house movies I've seen with unsimulated sex. You have Sada (female lead) going down on the male lead and then his cum dripping out of her mouth, plenty of fucking as well as a graphic scene with the woman 'laying' a hard-boiled egg the man sticks in her vagina. Even a forced lesbian geisha gang-bang for extra measure.
It's even available in high-def, thanks to the Criterion Collection releasing it on home video.