In a waiting room, I was reading that "fact book" which stated in Korea till the XXth century it was possible to enslave women but not men. That's the first time I'm seeing such a difference depending on gender. That obviously leads to two queries.
Is the book right? If yes, what is the story which led to such a rule? Was the rule respected or was it a case where theory and practice differ? If no, was there something which could explain the affirmation?
Was there such kind of rule elsewhere?
This is totally and absolutely incorrect. The traditional Korean word for slave, nobi, is itself a portmanteau of two Sino-Korean words, no ("male slave") and bi ("female slave"). The word itself should make clear that there were slaves of both genders. From the top of my head I can name the 15th-century inventor Jang Yongsil, the 12th-century would-be rebel Manjeok as two extremely famous examples of male slaves (I can't recall any female slave, for instance). There are countless paintings depicting male slaves.
Perhaps the book is thinking of the kisaeng, who were all female and slaves of the state, but