Marriage. Sex should be taboo and looked down on outside of marriage. No fault divorce should also be more heavily scrutinized.

None of this is rocket science. Society figured this out a couple thousand years ago when monogamy became normal even for royalty before/under Christianity. Of course some would try to make exceptions for themselves (eg Henry VIII), but they typically aren't held up as exemplary people today (or those particular actions against monogamy is viewed negatively, like with King David), and rightfuly so.