What do you say OG? I'm talking about major studio movies with big name actors.
A couple that come to mind for me;
Monster's Ball - Halle Berry & Billy Bob Thornton
Original Sin - Antonio Banderas & Angelina Jolie
Celebrity movie archive is a great website for such information by the way. 
Deliverance - Ned Beatty & Herbert "Cowboy" Coward
There was one in an HBO/Cinemax horror movie where this black girl had a hairy bush... It old now but when I was a kid it was awesome! 
Shower scene, American History X.
That sex scene with Jen Anniston. Oh wait, nevermind, i'm thinking of the sex tape we made.
The one with jolie & Ethan hawke
Latest 300 movie
Body of evidence- William dafoe and Madonna
Or
Monsters ball
the one where donald sutherland was really bangin that chick on screen. the title escapes me right now
^^^Don't Look Now?
Wet Hot American Summer. Bradley Cooper takes it up the pooper in like a camping shed. Can't watch his movies the same way anymore.
Joe Dirt "I'm Your Sister!" scene.
Requiem for a Dream
I can't believe Bound was not in the first three posts!
Mulholland Drive had a pretty epic lesbo scene. Both those chicks were smokin' hot.
What about the lesbo scene with mila kunis and the other chick in black swan.
"freeze frame "
it had an awesome F on M rape scene (no pegging, just riding).
Watchmen, night owl and silk spectre getting it on in Archie was pretty good
Is 'Chuck and Buck' mainstream??
I was going to post monsters ball. Billy Bob had at it