Board Thread:Questions and Answers/@comment-30741044-20171227153919/@comment-30741044-20171228015116

They could not simply "cast us aside" because once they have found a "husband" the mana AKA semen of other men becomes unpleasant for them. Their bodies also gradually change to better suit the tastes of their spouse so they can extract higher grade of mana (I'm just taking all of this info from the wiki, alright?). Sure,they can still fuck other men, but it would be like eating rotten food,and the quality of the mana is not that good.

In other words, they have no choice but to settle with what they in front of them,their "husband".