The demon "king" should've been his wife who OD'd or hanged herself after finding out he got splattered by a car.
After thoroughly defeating her with sex after a multiple chapter long "battle" (and therefore NTRing the other husbands a second time) they talk about what happened in the real world that led to this. The goddess says said the clear conditions for both parties have been met (defeat the demon king and save the world, find your reincarnated husband among these heroic candidates respectively).
The way it actually played out feels like it fell flat on multiple fronts. The messages it wants to impart feel flimsy and contradictory.