Vox is right.
Even if you want to tear it down to the very basics. The innate instinct to track down and find a mate is simply stronger in men (or it should be). All males, including mammals, birds, fish, reptiles, amphibians, and insects, are built with a need to reproduce. It is in the male nature. This need to reproduce is not as strong (although present) in females because technically, the guys 'make the babies.' It makes sense for a male to be more promiscuous than a female. He feels he has a job to do.
I'm not trying to give anyone an excuse because humans have taken things further than they should have gone, but we can't look at it simply. When a female acts irresponsibly and more like a man than a man, what are people supposed to think?
And still it all boils down to this, if you don't want to be called a whore, don't act like a whore. A guy can't get mad when he dresses up like a fireman and stands by a firetruck, and someone asks him for help, can he?