The point is that the state cannot, in Novak's view, create or recreate the social practice of marriage. The name marriage applies to a particular kind of relationship between a man and a woman. Again, he writes,Since traditional marriage already has reasons of its own, the state should not supply new and different reasons for an institution it only inherited and did not itself create. Indeed, to supply radically new reasons for something old essentially transforms it by radically redefining it. As such, marriage so radically redefined loses any essential continuity with what previously has gone by the name "marriage."
As a kind of practical apologia for this position, Novak goes on to argue that the best public reason for marriage is procreation, and the furtherance of the human race. This can obviously only be acheived biologically by heterosexual couples (de minimis non curat lex). Futhermore, since this is biologically the case, Novak raises questions about the legitimacy in a same sex marriage of a man 'taking the place of the mother' or a woman 'taking the place of the father' as would be the case in a same-sex relationship.Thus "marriage" did not become the name of the domestic union of a woman with a man because someone said, "Let there be marriage!" Instead, "marriage" became the name used to designate this already existing social relationship when it had to be distinguished from other social relationships that might look like it in some ways, yet are different from it in more essential ways.
Novak's position is a kind of defense of traditional marriage from nature (procreation) and from the tradition itself (the naming of a social practice). At one level it is a helpful and coherent argument that takes seriously the move in secular sociaty towards recognizing civil partnerships, but which rejects the idea that these partnerships are 'marriages', but on the other hand it makes a claim on the public practice of marriage and the reasons for it. In our, perhaps overly romanticized, world does this kind of argument from nature really carry the kind of weight that those who defend traditional marriage would like. For me the downfall of Novak's piece, and I suppose Nussbaum's (though I have read little of her work), is that it isn't theological enough - and doesn't ground marriage in a plausible anthropology.