So we’re stuck with this unfortunate irony: the institution that seems to be, on average, the least bad means of rearing children is an institution that doesn’t naturally sustain itself in the absence of moral sanction — positive sanction for fidelity, negative sanction for infidelity.
I don't think this is ironic, because I don't think marriage is a bond made primarily by our biological nature. Instead, I think the mother-child bond is natural. The mother-father bond, and therefore the father-child bond, is a great achievement of culture. Indeed, I think marriage and fatherhood are the fundamental civilizational institutions.
And civilizational achievements, like marriage, are made of moral sanctions.