It's amazing how the finer points of human interaction seem to be lost on a lot of people. Yes, the reason a lot of people, both men and women, go to bars/clubs is for an easy fuck with a stranger, and yes, approaching someone for sexytimes is okay in and of itself. However, if the current object of your attention makes it clear that (s)he is not interested in you, then back the fuck off and find someone else. Basic human decency still applies, after all.