Favorite sex scene in a movie?

"I came across time for you, Sarah. I love you, I always have."
The Terminator (1984)
Good choice, though I've seen it as a love scene rather than a sex scene.
In a film that is more or less one huge explosive action chase, most movies would probably fail big time by having thrown in some "random" hot and steamy sex scene, but Jim Cameron made it work to perfection.