Why is Fight Club a males-only club?

Tyler and the narrator came to the conclusion that the men in society are men raised by women. They have been emasculated. At Fight Club, they can come to be beaten down until they hit rock bottom. It is at rock bottom where they rediscover their primal instincts. Women, in Tyler's and the narrator's eyes, are the reason why men are castrated and need to re-learn the animal urge to fight.