Maybe mentioning a sexual body part is a step too far? It's like talking about a specific person's boobs. Just a bit too personal? Whereas telling someone they are like a cat in heat is a simile. Like they are like a white tornado.

I had a coworker who was ALWAYS going on about guys' cute butts. I never understood it. Turned out she was a lesbian. But maybe i need a course in sex education to explain why a guy's butt should be compelling to me!