I don't think most do. Actually the first time I've seen such a rule. Had to ask a friend before replying because he is in a very HC guild and he told me he doesn't even ask for your sex in the app because the GM doesn't see the point. You get judged in your trail for ability.
That being said, I can understand the rule albeit I don't agree with it. I've seen some serious drama in guilds get caused by women, the worst being a girl sleeping with the gm to get extra ep for loot (or whatever system they used) so being a male dominated game some women make take advantage as some do for everything if they can but in my experience in my guild, while not HC but were a serious raiding guild we had a female GM who was an above average player but the best GM I've ever seen in her ability to keep the guild together and the healing leader was female and simply the best healer I've ever seen.
Ofcourse there were also bad female players but there were bad male players as well. The difference is when it comes to ability the male player is bad because he's a bad player whereas the female is bad because she's a female. Sad but true fact in the gaming world.