Marriage started as a religious thing, but then became a legal thing. And that makes a lot of sense. In governing people well, government bodies have to arbit problems that arise. So they make laws by which to govern. That way, everyone knows what's the what. As regards relationships, there can arise many varied problems that need arbitration. Some relationships end, and all the couple-owned stuff has to be divided up. And, of course, children in the picture make it even more necessary to have a third, impartial party to sort out what's fair for all.
The detractors of Gay Marriage act all "holier-than-thou" (big surprise) about the Sanctity of Marriage, like they own the rights to it. They obviously care about the religious aspects of Marriage more than the legal aspects. Now, Religion is not for everyone, but the Law is. And the Law needs to be updated once in a while to reflect enlightened attitudes. Otherwise certain people would still be burning witches and owning slaves. (hmmm who would that be?)
Now no one can expect an Institutionalized Religion to change it's attitudes and policies regarding Marriage because People are free to worship however, and believe whatever, they want as long as they don't "break the Law". But we can, and should, expect the Law to give Marriage Rights (and responsibilities) to any two consenting adults who want to get married, regardless of ethnicity, gender, sexual-orientation, age (as long as they meet the minimum requirement), political and even religious affiliation.
Now is the time, and it's much past due. Let's get Humanity caught up with Human Rights "for God's sake" (heeheehee).