Also the administration is arguing that Prop 8 in California is unconstitutional
which is probably the strongest official support of gay rights ever in the US.
Obama was officially opposed to gay marriage very recently.
Also a lot of prominent Republicans came out in support of gay marriage.
Dozens of prominent Republicans — including top advisers to former President George W. Bush, four former governors and two members of Congress — have signed a legal brief arguing that gay people have a constitutional right to marry, a position that amounts to a direct challenge to Speaker John A. Boehner and reflects the civil war in the party since the November election.