If I'm gonna be a stickler about the lore, that XY chromosome combo seems to be a necessity for other memory of both sexes. Past that nothing says Paul can't be trans, intersex, etc.. Granted, by book 3 I think it's a little more loose on that requirement. This is also a book that has bioengineering and cybernetics. Still, Frank being an SF dude in the 70's meant he was ahead of his time in some areas, them wildly homophobic in others.

In short the powers the Space Nuns wield is regarded as too dangerous for men to handle, both being lethal to men who try to use it, as well as too dangerous if a man should somehow survive the training process.