unless you have a specific religion under which you desire to be married, all civil marriages are legal for all american citizens. to claim otherwise is unconstitutional, according mostly to the Ninth Amendment of the Bill of Rights. unenumerated rights are the most important rights american citizens have to protect them from the government and the "moral" majority, or any kind of majority.
marriage in and of itself is NOT a religious ceremony, unless you choose it to be. there is no need to "defend" something that is clearly available under the Constitution to all citizens. with a 50% divorce rate, "traditional" marriage doesn't seem to be very well defended, does it?