Well not being that entirely familiar with the zeitgeist of constitutional thought, and full technicalities on the matter in the U.S., I can only speculate.

But I would tend to think of it from this angle: is there any argument that could be made before the supreme court that state laws that prohibit gay laws unions are an unjust restriction on personal liberty? By which I mean, states are limiting people's freedom (or unenumerated rights, perhaps?) without reasonable justification?

Obviously the makeup of the SCOTUS would affect the likelihood of any such argument being presented and being accepted by the bench, if such an argument could be made.