Well, if they are exclusive or have criteria for joining they can discriminate all they want. Stinks, but they can. I've been denied membership in hsing groups simply on the basis that I am "not Christian" (as I've been told) or, rather, my brand of Christianity isn't "right" (I am LDS). Not much you can do other than complain (not that they'd likely care) if it's a private organization. They make the rules.