What gives anyone "Constitutionally" the right to decide if adults can marry one another or not. And where in the constitution would this power come from, since the concept of marriage is a biblical custom, and it just happen to creep it's way into US common law. For the state to set boundaries, "arbitrarily" on the gender of the individuals involved is a personal contract between consenting adults. The only cultural boundaries being crossed, are religious customs that provoke the conservative sensibilities of the faithful.
I think it is overreaching for the state to cross these lines further. The rights of all citizens are paramount and must be respected, and the happiness of any individual is their own, and not the business of the state or the community at large.