Why should we respond to SSM by having the state get out of marriage?

Over at Public Discourse, David Novak has a typically thoughtful take on same-sex marriage, and while I find the various elements of his analysis reasonable, I have a hard time connecting all the dots.  He argues that a child has a natural right to be raised by his or her biological father and mother.  The tricky part with this argument, of course, is what you do with adoption.  Here's how Novak handles it:

Despite all my talk about natural parentage and childhood, I am in favor of the institution of adoption. Surely, a child’s right to being raised by adults is better upheld by adoptive parents than by natural parents, when these natural parents are unable or unwilling to raise their natural offspring. And, in principle, I am not opposed to a gay or lesbian couple being able to raise a child whose natural parents have abandoned him or her (whether voluntarily, or necessarily in cases of death or debilitating illness). Surely, a child is better raised by a couple who love him or her and each other than being raised in the less personal setting of an orphanage, or being raised by foster parents who are paid by the state to care for children nobody else wants, and who do so at less cost to the state than the cost of maintaining orphanages. Nevertheless, all things being equal, I think it is best that such an abandoned child be adopted by a heterosexual married couple rather than be adopted by a homosexual couple. That is because a heterosexual couple can better simulate—perhaps improve upon—the heterosexual union that produced this child and should be raising this child. It better simulates the duty of the natural parents to this child, a duty they would not or could not exercise. This, by the way, is not arguing empirically that opposite sex couples are necessarily better at raising children than same-sex couples. My arguments are based on the concepts of rights, not on the concept of utility. Thus my arguments are a priori, not a posteriori.

And then he concludes:

I agree with [Martha] Nussbaum when she says: “I personally favor the solution of leaving civil unions to the state and marriage to religions and other private entities.” In fact, for me, such a move would greatly strengthen the social prestige of religious marriage. Yet neither of us is willing to give up on civil marriage, at least not yet. I suspect that giving up on civil marriage now would be an admission of political defeat neither of us is willing to make. In Nussbaum’s case, that would seem to be an admission that the institution of civil marriage cannot be reformed to ever really include all those she wants included in it. In my case, that would seem to be an admission that civil marriage can never be restored to its richer and more coherent traditional meaning. However, since this society is so divided on this question, the disestablishment of civil marriage altogether and its total replacement by civil unions could well be the way this society might have to go for the sake of civil peace.

Here's where I have trouble following the logic: Novak asserts that the public reason of marriage is to "facilitate procreation and the exercise of parental rights and obligations as well as filial rights and obligations."  If same-sex couples should be permitted to adopt children -- and indeed, if opposite--sex couples "are not necessarily better at raising children than same-sex couples" -- then why is the inclusion of same-sex couples so disruptive to the public rationale of marriage so as to justify disestablishing civil marriage altogether?  (I know he's not calling for that, but he is open to the prospect that it might be the most sensible of the viable options.) 

If the state has a role in supporting the commitments that are essential to stable and effective child-rearing, then why abandon civil marriage simply because the institution expands beyond its ideal form, especially since the new form, by my reading of Novak, still supports many (but not all) of the child-rearing goods promoted by marriage?  (Obviously, marriage routinely -- nearly always -- is practiced in some sort of non-ideal form among these fallen human vessels, though SSM expands the non-ideal form categorically, under Novak's argument.) 

Let me put the question somewhat differently: are committed same-sex couples who raise children together more like a traditional marriage, or more like any other civil contract with no capacity for, or inclination toward, the self-transcendence that should accompany marriage?  Even if SSM is deemed to be closer to the latter, isn't ending civil marriage altogether a bit like throwing the baby out with the bath water?  I can understand opposition to SSM, but I have a harder time understanding the argument that the proper response to SSM is to get the state out of the marriage business entirely.