Though gay marriage is a states' issue there are also questions at the federal level. For instance, should the IRS recognize a gay union as a legal marriage for tax purposes? And what about federal death benefits to the surviving spouse? A president will eventually have to take a stand on this issues and not defer to the courts to decide them.
No it is not... REYNOLDS v. UNITED STATES, 98 U.S. 145, 98 U.S. 145, October Term, 1878.
The federal govt has already taken a position on this. It doesn't recognize gay marriage. For example, the man recognized by the state of Massachusetts as the spouse of Gerry Studds was denied the right to collect the survivor benefits that his Congressional pension would have provided had he been married to a woman.