Alrigtht, I'm about 99% sure that Nishina either isn't actually the Jealouse god or didn't put the curses on humanity. Why would a jealouse god put a course on girls that can only be broken by fucking ON TOP OF coursing all men into being these Karu monsters from the start of the series... unless the curse was cast by a man.
It has been several thousand year, almost noone can read or interact with anything from the old world, ample time for false stories to be spread around and details to be changed.