Here's the premise I've come up with: Thooooousands of years ago, monsters were angry bastards who served their patron deities. The human deities had been killed by the others, because fuck humans. However, the trickster god kept fucking with everybody's shit. He especially loved boning the neglected wives of clerics and having them join his personal harem. The other gods get so sick of his cuckold crusade that they all team up to get him back for it. They invade his realm and through magical means, they turn the trickster god into a woman.

Trickster god thought this was so fucking funny that she laughed so hard she exploded. Her laugh echoed throughout the universe and turned all the gods' monsters into cute girls who want to have sex instead of kill things. The only ones spared from this were the humans, and so every other race now depended on the proliferation of humans for the proliferation of their own kind. The other gods were immensely fucking salty over this. Also, they were trapped inside the trickster god's dead realm, being unable to do anything about it.