Well have you guys thought it was something traditional or some crap you enherit
it would make no sense for it to be a tradition because that would imply that they get to choose the gender of their children which seems very unlikely.
and my second theory deals with the whole "inherit" thing except they dont inherit it, they are just created to only have one male every 100 years for no reason.
i agree with Bellum, though, i like my 3rd theory the best. it seems the most likely.