When I was in college, I was hanging out with a group of guy friends, and one of them—I’ll call him Paul—told a story about a buddy of his. At the end of a party, when there were people sleeping or passed out all over the house, Paul found his buddy having sex with a girl who was passed out drunk, unresponsive, and clearly unaware of what was happening. I say “having sex with,” but the technical term is “raping.” And the buddy says, “Hey, you want to try this?” And my friend telling the story says, “Nah. We gotta go.”
The reason that’s all he said, Paul told us, rather than, “What are you doing, you douchebag? Get the hell away from her,” was that he felt torn between his gut instinct that what his friend was doing was Seriously Not Okay and the automatic reaction of his body to the sight of sexual intercourse. He got an erection. He was horrified at himself, at the idea that any part of him might interpret this Seriously Not Okay situation as erotic.