I don't like it personally, but I fail to see what's controversial. It's a private business with rules. It's not very different from places requiring you to wear a tie. I can imagine some other establishment requiring you to bring a pet. A singles bar can require the opposite and exclude groups from entering. Part of the experience is socializing and I can see why some bars might want to have rules that cater to a specific audience.

If their rule was "no single men after 21:00" I could see that as a sex discrimination thing. But this is a perfectly reasonable rule, I don't like it or agree with it, but it's also entirely the business owner's right.