. Why is it different for woman to sleep around than a man?

A man has casual with many women and is dubbed as a stud, a hot commodity. A woman who has casual with men is considered promiscuous and "used goods". Could someone explain the discriminating dynamics of this contradiction to me. I don't see a problem as long as it's consenting adults, however I see a problem with the reputation each gender receives. How did such a miscalculation occur?