There are issues that matter on the most fundamental of human levels. Issues that truly divide opinion into two distinct categories. Issues where there is truly no moral ambiguity, no grey area. Just good and bad. Right and wrong. Marriage equality is one of these issues.
Be on the right side of history. Demonstrate the most basic elements of human goodness by extending equal rights to all Americans. Strike down the Defense of Marriage Act and Prop 8.
Ask yourself the question: how moral is the following? I am told of a human sacrifice that took place two thousand years ago, without my wishing it and in circumstances so ghastly that, had I been present and in possession of any influence, I would have been duty-bound to try and stop it. In consequence of this murder, my own manifold sins are forgiven me, and I may hope to enjoy everlasting life. Let us just for now overlook all the contradictions between the tellers of the original story and assume that it is basically true. What are the further implications? They are not as reassuring as they look at first sight. For a start, and in order to gain the benefit of this wondrous offer, I have to accept that I am responsible for the flogging and mocking and crucifixion, in which I had no say and no part, and agree that every time I decline this responsibility, or that I sin in word or deed, I am intensifying the agony of it. Furthermore, I am required to believe that the agony was necessary in order to compensate for an earlier crime in which I had no part, the sin of Adam. It is useless to object that Adam seems to have been created with insatiable discontent and curiosity and then forbidden to slake it: all this was settled long before even Jesus himself was born. Thus my own guilt in the matter is deemed “original” and inescapable. However, I am granted free will with which to reject the offer of vicarious redemption. Should I exercise this choice, however, I face an eternity of torture much more awful than anything endured at Calvary, or anything threatened to those who first heard the Ten Commandments.