Sure they can. It's discrimination based on sexual orientation, not gender...which is perfectly legal in a number of states.
Because that's where the best music is...and it's full of good looking men that aren't going to hit on you.
So let's see a bar put up a sign saying, "No homosexuals allowed", and see how long THAT lasts before there are lawsuits and protests and whatever.
And actually, btw, they're just ASSUMING those parties are for hetero women. What if a lesbian couple is getting married, and one of them decides to have HER bachelorette party there?
Why does this whining dipshit bar owner think he's "excluded from the experience", anyway? Who the **** is stopping him from having a wedding with all the trappings, including parties beforehand, if that's what he wants to do? Does he really need that piece of paper - and public approbation - so badly that he lets it stop him from living his life?