Because, paradoxically, a gay club can be a great place to meet women. Loads of straight women go to gay clubs to get away from the usual oafs that pester them in standard nightclubs and bars. And also, if you're on a night out with gay friends then wouldn't it be slightly rude to refuse to enter a club purely on the fact that it's a gay bar?
I've picked up more women in gay bars than anywhere else combined. It's fantastic for that.