A lot of people assume this is a double standard enforced mainly by men. Which is completely untrue. Think about what you've seen personally and you'll probably realise that it's rarely ever men who call or consider a woman a slut, or make her feel bad for being free with her sexuality. It's usually fellow women (I stress "usually" here, just for the record. You do get the possessive man types who like to throw the word "slut" around). You've all seen and heard it. For some reason it's a big thing in modern western culture for women to treat each other, frankly, really really badly. So if you're wondering where any of these double standards come from, that's what you really want to be investigating. The old days of women being oppressed by a male-dominant society are over. These days women's biggest enemies are quite often themselves.
EDIT: Might as well throw in an opinion on top of an observation. I don't care how many guys you've screwed, as long as you're clean and I like you anyway, it's all good.
EDIT: Might as well throw in an opinion on top of an observation. I don't care how many guys you've screwed, as long as you're clean and I like you anyway, it's all good.