Richard Socarides, writing in the New Yorker, explains why it matters that the President came out in favor of gay marriage.
[A] year from now we will likely be on the verge of an important ruling which will articulate, either narrowly or expansively, the Court’s view on the rights of gay and lesbian Americans to full equality.
Now that the President has personally endorsed marriage equality, his own victory or loss in the election will be important to the outcome of these cases. (So will the marriage referenda on ballots in the fall—Maine, Maryland, Minnesota, and Washington.) No matter what anyone says, Justices make decisions in a political context, and if he is defeated the Supreme Court will see it as an implied repudiation by the majority of Americans of his position on gay rights. On the other hand, if he is reëlected, especially by a comfortable margin, it will assure the Supreme Court Justices that being in favor of gay rights, and gay marriage in particular, is not institutionally toxic. (Amy Davidson posted about this after the recent court decisions.)
It also matters vitally that American cultural leaders continue to express public support for marriage equality. This is especially so for “unlikely suspects”—Republicans and independents who otherwise have more traditionally conservative views. Every time a Bush or a Cheney, or a sports figure or Wall Streeter, says they support the right to marry, others will give it new look, and many will come along. All of this helps to create a political environment where gay rights are seen as mainstream (because they are now).