She's just phucking with him with the book ...probably?
Probably not. That concept isn't just in this series, or even just in the martial arts cultivation genre. Much of the world has come up with similar ideas at various points because, well, a pairing of a man and a woman can even create a new life and all. As far as all kinds of spiritual and occult shit goes it's not really surprising that it would end up being representative of power in some way.
In the present, you can probably find people practicing tantric magic rituals, because the idea exists so naturally there are people trying it. Basically, sex fueling some sort of spell (whether or not it might actually work being debatable), which may or may not most often just be an excuse to talk each other into fuckin'. "It will help us generate power." "Oh, well, if we must."
'Course, this is copping out on it anyway. They just have to be naked and cultivate back to back.