I'm surprised there's no NTR tag. After all, in essence, what is going on here? - The wife is cheating on her husband behind his back with some inferior, retarded, underage bastard. If she had been the least bit more mannered and noble, she wouldn't have allowed this to happen, even if it were an eternity without sex, because honour comes first.
What about the other two girls? They're pretty young, but will they know something about dignity and be virgins?