A good one is "Fruits of Passion" with Klaus Kinski. It is a French movie with an English translation. In one scene a Chinese torturer is told by Klaus to stand in front of a beautiful young girl. She gets on her knees, takes out his cock and starts sucking on it. It was filmed showing everything explicitly. And no, it was not a porn movie, but mainstream.
Another one is "Romance", where there are a number of explicit sex scenes in a mainstream movie.
Another is "Baise Moi", where there a number of explicit sex, rape scenes, and guns + sex in a mainstream movie.
Then there is the banned "Emanuelle in America", which takes Emanuelle into the hardcore world. This one has lots of violence and is too cruel for me and 99% of the population.
Of course there is Caligula, with its mind-blowing sex scenes.
"Brown Bunny", an American movie with explicit fellatio. Meg Ryan goes down some stairs into the basement to witness a girl blowing a guy.
"Betty Blue", a mainstream French movie with some explicit sex scenes.
"In The Realm of the Senses", a 70's Japanese film about a man and woman holed up in their house. Completely concerned with sex (explicit).
"The Idiots" - a Dutch film about patients in a mental ward and their care givers. There is a scene with an orgy that shows full penetration. Some of the patients are involved.
There are others, but I don't remember their titles. Anyways its interesting when the border between hardcore and mainstream becomes blurred. I hope it becomes common place - would make going to the movies more interesting.