My 60 year-old father, the man who took me to a Reagan/Bush 84 rally at the ol' Madhouse when I was 5, is a judge. No weepy liberal is he, but he is fond of pointing out that gay marriage is coming and all of the Falwellian hand-wringing in the world isn't going to stop it. This is so not because it is possessed of inherent moral "rightness" or any such nonsense but because the law is firmly on its side. Civil law, that is (religious definitions of marriage being another story altogether). Gay marriage is in fact the most perfect example of how poorly conservatives understand the Constitution. If only they devoted as much time to reading the damn thing as they do to "defending" it.
Most legal arguments over gay marriage, especially at the state level, tend to revolve around equal protection claims – 14th Amendment stuff. However, Article IV, Section 1 of the Constitution (available here, TeaTards) contains something more relevant called the Full Faith and Credit clause. It states:
Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State. And the Congress may by general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof.
The FFC is the reason why it is not necessary to get a new driver's license to drive in a different state. Every state in the union accepts my Georgia-issued license as valid, as valid as if their own state issued it. I am legally married in Marion County, Indiana. I did not have to get re-married to be legally married in Georgia. Georgia is required to respect and give credit to the public "acts and records" of Indiana. Similarly, if I legally adopt a child in Georgia, said human is legally my child in every state in the union. States are not required to give Faith and Credit to laws passed in other states. For example, a gun can be legal in Idaho and illegal to own in California. Buying fireworks in a state in which doing so is legal will not keep you out of trouble if you bring them into a state in which they are illegal. So laws can differ by state but respect for the "acts" and "records" is guaranteed among states. This is quite clear.
The Defense of Marriage Act (DOMA) explicitly contradicts Art. IV, Sec. 1 of the Constitution. Anti-gay marriage groups always pull back on their legal challenges of pro-gay marriage laws because they have no illusions about how the Supreme Court is likely to rule on DOMA. Sure, Clarence Thomas will engage in some mental gymnastics and Activist Judging to justify the law, maybe Alito too. But who else? To argue that the law is constitutional requires one to accept the validity of the FFC clause while allowing Congress the authority to define what is an is not an "act" of state government. But from where does Congress derive this authority? If we read that ol' Teabagger favorite the 10th Amendment – Seriously guys, try reading this thing at some point! All kinds of wild shit in here! – we notice that:
The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.
If anyone can find the part of Article I that delegates the power to define what is an is not an Article IV "act", please bring it to our attention. Other than some weak-ass recourse to the Elastic Clause, there is none…which explains the non-Libertarian right's hesitance to appeal DOMA challenges in the Federal courts. They prefer the mob rule approach, seeking to outlaw gay marriage by ballot referendum or by amendments to state constitutions. Legally, a state has every right to declare gay marriage illegal or unconstitutional. What they do not have is the right to ignore or nullify legally constituted acts of another state absent the ridiculous DOMA law that can't hold its own in any court except the court of mid-1990s public opinion.
If a single state is issuing gay marriage licenses, those licenses must be recognized by every state. The Full Faith & Credit clause is clear on that point and wingers are ever so fond of reminding us that they follow the Constitution literally. For the same reason that quickie divorces used to be a major industry in Nevada – those divorces had to be recognized by states with much more complex divorce laws – one state with legal gay marriage is enough. This is what any Federal court deciding a DOMA challenge on the basis of the law and not moral/political hysterics will say, which in turn is exactly why Rev. Dobson and his ilk always stop their legal challenges just short of being told what they can't accept and don't want to hear.