The ironically titled Defence of Marriage Act was implemented to restrict marriage to opposite-sex couples in the face of a surging momentum to sanction same-sex marriage in various states across the union, including California, Washington and New York. Section 2 of the act says this:
No State, territory, or possession of the United States, or Indian tribe, shall be required to give effect to any public act, record, or judicial proceeding of any other State, territory, possession, or tribe respecting a relationship between persons of the same sex that is treated as a marriage under the laws of such other State, territory, possession, or tribe, or a right or claim arising from such relationshipToday's decision strikes the law down but, and this is an important nuance, does not "legalize" gay marriage. Saying you can't ban something is not the same as saying that it's legal, and I expect that the struggle for equality will be fought on a state-by-state basis. The National Journal has a very useful map showing the state of gay marriage across the US.
As a footnote, it occurs to me that if one were really to "defend" marriage, one would ban arranged marriages and provide couples planning on marrying with counselling and information on the legal consequences of marriage and divorce.