So there was a comedy-ish novel where our demon king finds it annoying that the neighbouring kingdom uses religious bullcrap and sends assassin's (heroes) at him when he only wants to govern his domain properly. As punishment he turns hero into his wife after changing his gender(lol).
After checking my "not yet read" list I think I found it https://www.novelupdates.com/series/maou-sou-da-yuusha-wo-yome-ni-shiyou/