Can you name any sex scenes that actually provided DEPTH to a film, and wasn't there for gratuitous sake?

I will always remember the sex scene in 300: Rise of an Empire not because it's got Eva Green in a sex scene (rhyme not intended), but because the scene in nature actually provided something towards the story and wasn't there for the sake of it, like most films do.Here are two people trying to prove they have more power than the other, and they do it through sex. It's dominating, raw and powerful, and it's a test to see who can wield power better. It's a great scene, from a story point of view, because it isn't there for gratuitous sake.So that brings me to the question: has there been other examples in film where sex has been used to further the plot, or provide depth to a story (like the aforementioned scene from above)?The scene must provide depth / character development / further the plot / there for a reason andNot: there for the sake of it / could have easily been cut from the film and no one would blink an eye / oh look, it's two of the biggest stars doing the sexEdit: Whoa, wasn't expecting this many responses. Awesome examples, guys! via /r/movies http://ift.tt/2lsfXQY