Perhaps the most depressing and uncomfortable sex scene in film history belongs to Darren Aronofsky's 'Requiem for a Dream.' Jennifer Connelly's character is desperate for heroin and nearing rock bottom when she decides to call up Keith David, who will give her heroin in exchange for sexual favors. She thinks it's harmless enough until she shows up at his apartment, where Connelly is expected to double-team a sex toy with another desperate woman while drunk, wealthy men throw money and yell at them. The way Aronofsky films the scene creates the illusion that Connelly is actually participating in the sex act, and while it's painful enough to watch, her sweaty, pained face makes things extra uncomfortable. The footage is woven together with the climactic scenes from the other lead characters' stories, making this sequence intense and cringe-inducing.