Mother Panda, I've been thinking about this a bit. Feminists have worked long and hard to get the point across that rape isn't about sexual desire gone overboard.
Rape (and I'll include serious sexual assault, as UK law restricts rape to penetration with a penis without consent) is about power and control. It is one person exerting power and control over another person by engaging them in a sex act without their consent.
Man A wakes up in the night feeling fruity. His partner is completely sparked out (i.e. asleep or passed out drunk.) He proceeds to have sex with her without her consent and/or doesn't stop when she says she wants it to stop. Man A feels he is entitled to engage in sex with his partner, whether or not she clearly consented. He is exerting his power and control over her body and her person by engaging her in sex without her consent.
Man B on the other hand wakes up in the night feeling fruity. His partner is completely sparked out. If asleep, he might decide to wake her and see if she wants to have sex. If passed out, he knows she's in no position to agree, so turns over and has a wank.
Both men were aroused and in the mood for sex. One raped and the other didn't.