Is this immature and inconsiderate of women and does it reflect poorly on the LAN? Absolutely. Are they within their right to do so? Absolutely.
People get up in arms about stuff like this because they're offended that people would so boldly exclude someone, but the key here is that it's a private event. Don't like it? Don't support them. Or, create your own private event with any rules you see fit.
As a nonreligious white male, I'm also excluded from minority scholarships, many religious private colleges, jobs with many religious organizations, being a waiter at Hooters, and plenty of other social groups, simply because of my personal beliefs or race or gender.
Women also can't join the Augusta National Golf Club (home of the Masters) for similar reasons as this, but I don't see that on the frontpage of HN.