Straight people tend to get offended when a gay person hits on them, and drunk offended people can sometimes get violent. On the flip side of the coin, heteronormative bars present an added challenge for gay people looking for a hookup as they can't always tell who will be receptive. There was a need and the market filled it.
And for the record, straight people are allowed in too, it's not like they scan you with a gaydar at the door. You just have to be ok with people hitting on you who you're probably not interested in. So it's like being a woman in a normal bar.