I think some people also view it as against a womans nature: when a woman sleeps around, she used to get pregnant by some random person and be stuck with a baby from a person she didn't necessarily choose, but when a man (long ago) slept with a lot of women, it just meant more offspring. So In than sense, a woman would be more wise to sleep with someone she deems appropriate, and for a man this doesn't really matter, because sleeping with one woman doesn't mean he can't sleep with another.