I don't have time here to fully develop this idea, which really requires a full essay. But, just to get the point out there..... James Taranto notes a weakness -- and an unforgiveable one, because Justice Kagan's question was one of the most anticipate-able questions in all of oral argument -- in the presentation by Prop 8 (anti-homosexual-marriage) lawyer Chuck Cooper. In short, if the only really good argument in favor of limiting marriage to its traditional male-woman understanding is the argument for procreative purposes, then that makes it invalid for couples who are infertile for reasons of age or other health problems. The trad-marriage defenders long have made the mistake of putting all their eggs (so to speak) in this basket. It's a very important argument, and indeed the primary argument... but it is not all-encompassing.
The fuller argument would go beyond procreative considerations into the broader point about the very nature of gender differences that are, despite Leftist theory to the contrary, very real. Those gender differences, mainly but not entirely physical, are substantial, and they exist regardless of how people "feel" about each other.
In short, the trad-marriage argument is based on the complementarity of men and women -- a complementarity that cannot be denied, no matter how much the gender blenders might want to pretend otherwise. The procreative aspect of this complementarity is surely the most important aspect, from the standpoint of societal benefits, but it is not the only aspect of it.
Traditional marriage recognizes this complementarity and serves as a building block of stable societies because of it.
I invite better thinkers than I am, with more time available, to more fully develop this idea in a way that might appeal to the reason of the Supreme Court justices.