Manzi rushes to the defense of matrimony:
There is no practically-available social scientific method that can provide the kind of proof that Marcotte demands in a non-totalitarian society.
But it doesn’t follow that those who advocate continued legal support for traditional marriage are, as Marcotte puts it, “marriage chauvinists.” As I’ve written about elsewhere at length, I think the general principles for political action that flow from such a lack of scientific knowledge on a topic like this are: (1) a loose status quo preference, and (2) a regime of subsidiarity.
I'm with Jim on this, for what it's worth. It's why I support marriage equality. Why dream up a new institution - civil unions/domestic partnerships/civil partnerships - when there's a perfectly good one sitting around?