Frankly I have no horse in this race, nothing personal in any case. For my money you may marry your grandmother or cat, if all parties consent.
Marriages ought to be a matter of contract and not based on any myth or superstition. Folks should not be interfered with if they want to form a family union, however it is configured, so long as it isn't some kind of criminal gang.
What I do find odd is for the White House to butt in here, requesting that the US Supreme Court invalidate various state statutes to conform to the doctrine of the ruling party or the president. In the case of President Obama, an avowed champion of democracy in numerous areas – whereby as far as he is concerned, majority rule may violate individual property or contractual rights (so that, for example, he supports imposing all kinds of burdens such as various taxes) on everyone because the majority agrees – the demand that the court uphold the ban on gay marriages would appear to be perfectly acceptable to him if the majority in a state, such as California, so decides. But, alas, democracy must yield when Obama so wishes.
Democrats, be they lower or upper case types, often do not get it: If you believe that what the majority agrees to should be the law, you have no cause for complaint when insidious measures get passed in various elections in various jurisdictions. Majority rule means just that, rule by the will of the majority. If you think there are exceptions – as even the US Supreme Court has said there are and as most sane people would agree – you need to show why. The best case for them would provide support from the political doctrine of natural individual human rights. So that if everyone has a right to speak his or her mind, no majority would be authorized to shut us up no matter how outrageous our ideas happen to be. And there are other equally well-established individual rights that no majority ought to be authorized to breach. So, yes, whom one chooses to marry if all parties agree (remember polygamy!) may not be subject to interference by a majority or its representatives. Don't like it but live with it, if you have any respect for the right to individual liberty!
But then do not impose on people measures, laws, regulations, etc., they find morally or otherwise objectionable unless these amount to protecting individual rights! But it doesn't seem to me at all that Mr. Obama and his ideological cohorts have any firm commitment to such individual rights, only to some select ones that happen to suit their pragmatic frame of mind. In other words, they are essentially committed to a fascistic type of "legal" order wherein those who happen to sit atop the government get to tell everyone else what goes.
And they used to fancy the US a free country! Go figure.