Douthat surveys the potential points of impact, variously, that same-sex marriage will (a) make same-sex relationships more conservative and faithful than they have been heretofore; (b) make all marriages less faithful; (c) provide the means by which the relative faithfulness of a marriage will be negotiated by the partners.
Fair enough: we don't see any other likely permutations. But then Douthat gets the the heart of the matter: America's prosperous classes experimented with swinging a generation ago (as Updike chronicled, by the way), and discovered increased misery. Hence, marriage among the better off has become more stable most recently.
So here's the lesson that ought to be learned, but won't be learned often enough or deeply enough:
Institutions tend to be strongest when they make significant moral demands, and weaker when they pre-emptively accommodate themselves to human nature.