Double standards of promiscuity?

In most of our societies, you've probably noticed, a certain double standard, when dealing with the promiscuity of either gender. Men are rather expected, it seems, to have a number of conquests and are often judged on this number, whereas for women, we're more likely to simultaneously be expected to remain celibate until marriage as well as put out for men. If a man is promiscuous it is expected and pretty much acceptable 'normal' behaviour, whereas if a woman was to act as equally promiscuous as such a man she would be called numerous slandering names such as 'sl*t', 'wh*re', 'sl*g', 'tart', whatever. Why do many people seem to think it acceptable behaviour for one but not the other, and why do many people find promiscuity so unnacceptable in itself? What I really don't get is when some men preach against promiscuity in only women-how can men be promiscuous but women cannot? Surely it takes two to tango? If women refrain, who do men plan to have with? Each other?