Wow, some of you guys are just...dang, completely clueless. Not all men are sex crazed freaks who feel the need to constantly proposition women. In fact, if you knew much about women, you would know that most women much prefer the man who doesn't need to proposition them to the one who does. Cool, calm confidence will go a lot farther than idiotic frat boy bravado.
If a woman walked up to me and asked me if she wanted to do the horizontal mambo, I would back away slowly until I got far enough away to run quickly. That kind of "rawr, get with me now you sex maniac" is not attractive to me nor is it to a lot of men who aren't completely desperate for their next big score. Nor do most women find it attractive when men have the same attitude. Desperation is generally not a sexy quality.
This has very little to do with women "not spreading their legs", which is a total fallacy and almost offensive. This has to do with women, and many men, being selective about their sexual partners and not just whippin' out for whoever comes calling. It's easier for women to get laid because, as a whole, men are far more desperate to get laid than your average woman. Completely missing the point that it's their very desperation that is keeping them from finding a worth while partner. It's that "I'll stick it in whatever hole presents itself" attitude that gives men a bad name, and gives women that creepy feeling when you try and eye them up at the bar.