But, where in the O.T. is this boy's actions prohibited?
It's not adultery. And, concubines were apparently acceptable.
Just curious. Could you point us to a verse?
There is a verse that I think is in the OT that says something to the effect of it being a sin to "waste your seed in the belly of a whore". I don't remember the exact wording or exactly where the verse is, I just remember it because it sounds so gross.