It was landmark U.S. Supreme Court precedent Reynolds v. United States in 1878 that made separation of church and state a dubiously legitimate point of case law, but more importantly; it confirmed the Constitutionality in statutory regulation of marriage practices.
"Laws are made for the government of actions, and while they cannot interfere with mere religious belief and opinions, they may with practices So here, as a law of the organization of society under the exclusive dominion of the United States, it is provided that plural marriages shall not be allowed. Can a man excuse his practices to the contrary because of his religious belief? [98 U.S. 145, 167] To permit this would be to make the professed doctrines of religious belief superior to the law of the land, and in effect to permit every citizen to become a law unto himself..."
(see also: United States v. The Church of Jesus Christ of Latter Day Saints.)
It did argue that federal law trumps religion, despite the 1st amendment, which, when you think about it, tends to weaken the traditional marriage argument. The fed court may indeed use Reynolds to solidify gay marriage, if the only argument against it is based on religion.
Marriage is one of the "basic civil rights of man," fundamental to our very existence and survival.... To deny this fundamental freedom on so unsupportable a basis as the racial classifications embodied in these statutes, classifications so directly subversive of the principle of equality at the heart of the Fourteenth Amendment, is surely to deprive all the State's citizens of liberty without due process of law. The Fourteenth Amendment requires that the freedom of choice to marry not be restricted by invidious racial discrimination. Under our Constitution, the freedom to marry, or not marry, a person of another race resides with the individual and cannot be infringed by the State.
Notice it says the freedom to marry or not marry another "person." Ruh-roh. I sure hope the libs on the SCOTUS haven't read that one!
Here in NJ, I vote against gay marriage whenever it comes up. They did pass civil unions here. As far as I can tell, it's a state matter. I don't see the delegated power in the Constitution. And while those of us in NJ or MA may not like the outcome, leaving it to the states is superior to consolidating the whole matter under federal authority. The outcome there will be imposed on everyone from Maine to Mississippi, from Washington to Florida, and if the final decision comes from the SCOTUS, it will be without appeal. Be careful what you wish for.