As a straight man (who has a few gay friends, and has been to a few of these bars in the past) I understand why they do. A lot of men are predatory creeps and you're far less likely to be hit on by such a creep if you go to a gay bar. But then you're gonna see more straight men show up and start hitting on women who likely won't be interested because of their sexual orientation.

That being said, I have met some very predatory gay men (mainly outside of these bars), and a lot of gay people I've met previously have had experiences of being spiked. I'll just leave it at that.