I've raided as off-tank during Naxx 25 (WotLK) and the main tank was a woman, she rage-quit for the night because I pulled TRASH mobs while she was on her 4th bathroom break for the night... She was in a "relationship" with the guild leader so most ppl went with it... I quit the guild 2 days later and met a woman that was playing a resto shaman and she was amazing, she was role leader (during Ulduar time) and she helped me and other new guild members with lots of raid and non-raid things.
I guess you could come to the assumption that a GOOD female gamer is equal or even better than male gamers, but a bad temper female is enough to make rust want to go away from your car. I mean... there's nothing worse than a drama queen looking for attention and justifies it because she's a "girl". Because of the last part there (with the drama and shit) I can see why some guilds preffer not having women in their guild, though this "no women allowed" policy isn't the strangest I've seen... I've seen Dwarf, Gnome and Tauren only guilds, RP realms ofc but to the point they wouldn't even interact with players that were of a different race...