I have a feeling they don’t see where the immorality of monogamous premarital sex is supposed to be (Where’s the treachery? The sociopathy-level objectification?)–and they certainly want a more substantive response than “Because God says so”. They’ll certainly want to know why God forbids it, or at least discourages it. (And something beyond paternity certitude, if you would. The French nobles of the 10th-13th centuries were fools, anyway.)
“God’s ways are not our ways” doesn’t work here, anyway. That was referring to El not being as short-tempered as your typical Levantine or Mesopotamian ruler, but genuinely appreciating the role of Rule of Law over Rule of Will (or thereabouts…). He is not supposed to be incomprehensible to a degree one would normally expect of Cthulhu, Nyarlathotep, Hastur, or Atlach-Nacha.
(And Lust is the least of the Cardinal Vices, not the worst. That would be Arrogance. As in the first horseman of the Apocalypse of John.)