Rule #1 of Harem Life: Once the number of Wives reaches critical treshold, you will not be in charge anymore, but they will be.
That treshold is at best, 4, but can easily be as low as 2.
Cain might as well stop resisting... The final number of wives he will have will be decided by the consensus of the Order of Fiancees, and there's nothing in the world he can do about it, except run like hell into an alternate world, and hope the Wives don't find him there.
Horny Princess doesn't even need to love him, and won't care...
Cain is at this moment roughly a Demigod, and Apostle for this world's Creator God.
Demons value strength above anything, it's in their genes...
Cain is by far the stongest being on this world, and she will have his babies.
She can't help it. Cain is pure catnip to her...