My favorite important sex scene in film was in Twelve Years a Slave. It established the nature of chattel slavery and the relationship between two random slaves. I was drawn into the true reality of the Antebellum South by watching the fucking slaves fucking. It was an essential part of the film, and fun to watch! I'm also quite sure it was faithful to Chiwetel Ejiofor's original autobiography even though I haven't read that because reading is for nerds.