Late to the party, but I wanted to chime in on this very sensitive issue anyway.
First up, this
Wikipedia link on marriage licenses in the U.S. with a brief but very good history behind the requirements. It should be noted, however, that prior to formal requirement of a marriage license as we know it today, cohabitation via "common-law" was the accepted norm among the states in this country. In fact, common-law marriages are still accepted in many states. For that reason, I find it very fascinating that so many people - politicians, as well as ordinary citizens - got so wrapped up over this marriage issue. Now, to the heart of the matter...
The Supreme Court ruling really wasn't about divining marriage contrary to how some are and have framed the argument. It really was about equal protection under the law. Yes, I know some folks who have posted to this thread think otherwise, but that only illustrates how they aren't thinking the matter through. The case that was before the Supreme Court dealt with a very simple matter:
"How is that a couple who has been lawfully web in a state where gay marriage is legal not be allowed to their spouse's death benefits under Social Security since DOMA does not recognize marriage between two members of the same sex?"
As most folks know, DOMA - the Defense of Marriage Act - stipulates that the federal government will only recognize marriage between a man and a woman. However, marriages are neither "licensed" nor "sanctioned" at the federal level. Their legality takes place as the state level (licensing). Moreover, with the exception of common-law marriages, most if not all are sanctioned by the church (regardless of religious affiliation). So, what you really have here is a state's rights issue combined with a "separation of church and state" issue. With this in mind, the Supreme Court had no other choice but to declare DOMA unconstitutional because:
1. Marriage legality has long been "licensed" by the states;
2. Under federal law, the surviving spouse is legally entitled to claim the survivor's benefits of their deceased spouse (Social Security); and,
3. Most marriages are sanctioned by the church, not the federal government.
It really is that simple.