It's not necessarily impolite to say, "this is our group, and we don't want just everyone to join us. I'm afraid we don't want you." There may be reasons for such. I don't take offense to the women of our church having female-only gatherings.
On the other hand, if they sold tickets to people, and didn't tell them beforehand that certain people wouldn't be allowed, even w/o tickets, that's another kettle of cis-gendered fish.