Adam McKay has called for a space where mad-dog rhetoric might give way to reasoned exchange and I want to take on a specific question of his in that spirit. I fully support gay marriage myself because I think of marriage as a social contract between free individuals—and that's all. If I thought of marriage as an institution into which free individuals choose to enter in order to fulfill a divine plan, it would be a different story.
Forget scripture. Consult the copy of John Locke's Second Treatise of Government (1689) you keep on your desk next to The Federalist Papers. Focus on what Nature looked like to a rational observer who had no notion of evolution but was confronted with all these marvelously adapted creatures (as in "created") dwelling in a Newtonian universe. It looked designed. Wings for flying, teeth for chewing—not because of mutation and natural selection, but because that's how the Maker made them. Ditto for all the behaviors we now think of as instinctive, including mating behaviour and nurturing behavior. Under these conditions, the traditional family looked like a natural arrangement.
And that's how Locke saw it. But, being a forward thinking modernizer with an interest in social contracts (like constitutions), he also saw marriage as a contract that, in effect, institutionalized that natural arrangement—in rough analogy with the way the founders thought of the Bill of Rights as institutionalizing the individual natural rights with which our Maker endowed us. Locke thought of our Maker as our owner, by the way. Things like murder were ultimately wrong because they were robbery of God, not because people had rights to Life, Liberty etc. apart from God. That's how deep this "divinely ordained natural plan" idea goes in our historical template.
So that's why religious conservatives experience gay marriages as threats to their marriages. When they get married they think they are entering the institution I just described—and gay marriage is a threat to that, obviously. Of course, the secular liberal wants immediately to respond "OK, go ahead and think of your marriage your way and I'll think of my marriage my way." But, to a religious conservative, that literally means that marriage isn't really a divinely ordained institution at all. The logical implication of that compromise is that marriage is what free individuals choose to "think of" it as—and we are back to the "relativism" conservatives like Pope Benedict are always so eager to denounce.
And we can see why. We can also see why both sides feel as if the other side is "imposing" its beliefs on society. I'm afraid we can also see why these values issues are not going to go away. Finally, we can see how important it is to keep evolution front and center in the biology curriculum. You don't have to be a relativist to defend gay marriage against the religious right. But, if you aren't, you will need a foundation in nature of your own...