It's threatening, b/c that means guys would have to deal with women sleeping with as many partners as they have had or they would have to deal with being rejected by a horny woman. Some guys don't like to be with a woman that has had more sexual experience than them, AND they like to stereotype that sexual women will sleep with anyone and everyone.
Saying that females don't like sex as much just makes them feel better.
I know not all men are like this, but I'm talking about the men that do find it threatening.
Also, some females might find each other's sexuality threatening b/c they're worried about a pretty and sexual woman stealing their man.