Like I said yesterday here and here:

But while Mr. Bush’s response had political clarity, it left supporters on both sides of the issue puzzled as to the legal aspects. The reason is that there already is a law, known as the Defense of Marriage Act, that appears to address the two principal concerns of gay marriage opponents. The law, signed by President Clinton in 1996, prohibits any federal recognition of gay marriage, meaning that benefits like those given under Social Security or to veterans may be claimed only by a surviving spouse of the opposite sex. In addition, the law relieves states of any obligation to recognize gay marriages performed in other states where they might be legal.

Source: Bush Backs Bid to Block Gays From Marrying