As Seen In: I Spit on Your Grave (1978)
The film's defenders will label this scene as the ultimate depiction of female empowerment, since Jennifer (Camille Keaton) is avenging her own vicious rape. In a film this ugly, nothing is that easy, though, which is why prefer to think of I Spit on Your Grave's coarsest scene as just plain nasty.
Matthew (Richard Pace), one of Jennifer's rapists, thinks that she's not mad about what he and his buddies did to her, and welcomes her efforts to masturbate him while he's taking a bath. As he nears climax, his closed eyes prevent him from noticing that Jennifer is about to cut his dick off. Which she does.