To me, the most striking moment in the Federalist Society's online debate is when Amy Wax, a law prof who argues that gay promiscuity will undermine the norm of monogamy in marriage, backs herself into a corner where she says this:
One also has to ask -- why is same-sex marriage so unpopular with voters? I think they see that once we start redefining, all bets are off. And I actually think that all bets ARE off. In the end, marriage is arbitrary, a construct, and a restrictive one. So why have it at all?
In context, it's fairly clear she doesn't mean the question rhetorically. She doesn't know the answer and she doubts there is one.
So marriage is arbitrary. It stands on nothing but blind acceptance of tradition. No moorings at all. Thus do conservatives, in their determination to put marriage on a slippery slope, join radical egalitarians in pooh-poohing the idea that it has any coherent rationale. Rad-egals say, "Marriage is arbitrary so let's change it." Conservatives, "Marriage is arbitrary so we can't touch it." Take yer pick; we're all deconstructionists now.
With friends like that, it's no wonder marriage is in such precarious shape.