I think it could be internal stereotypes... I personally feel like if girls wanted to do that then they could.... I mean, I hear way more women boasting taking a man's virginity than anyone else. Maybe a guy wants to do what they feel women can do, and blatantly, it takes two.
So maybe people need to stop taking the promiscuous male as attractive as guys have been wary of the active female for ages now. Rightfully so.
I'm a guy, but I don't really know. I haven't really experienced the urge to sleep around when I'm dating someone (including now thankfully), although I guess I really don't feel that way even when I'm not dating anyone. I've kissed a couple of different semi-strangers before while I was single, but that's about it. It probably just depends on the person's personality for the most part.
Speaking from a biological standpoint, men are pretty much designed to have an urge to 'sleep around'. It was the way to ensure our survival as a species many years ago (more sex = more babies = higher chance of surviving as a species)... So to answer your questions, it's just human nature.