If what you're saying is that all decisions should be made by popular majority, I respectfully disagree. If we suddenly decided to operate under direct democracy, I am confident I would not be pleased with the results. I am also confident most conservatives would not be pleased with the results.
Virtually everything about Western civilization has changed, in fundamental ways, in periods of a lot less than 5000 years. That marriage being between a man and a woman is one of the few things that have survived is probably the reason many people are so adament about preserving that paradigm.
But in my view, most of the ways in which Western civilization has changed, and continues to change, have a lot more practical consquences for the future of our society, and the individuals of which it consists, than whether two guys can be married under the law. In other words, if our nation survived revolution, civil war, abolition, reconstruction (Jim Crow), universal suffrage, a multipolar political system, two World Wars, a bipolar political system (the Cold War), the transition to a unipolar system (the fall of the Soviet Union), and the Macarena, I'm not sure how to view the argument that two guys getting married is going to be the last straw.