Politico says:

Outgoing Secretary of State Hillary Clinton remains the biggest-name potential Democratic 2016 presidential hopeful who has yet to take a position in support of gay marriage, a source of discussion in Washington this week after the State Department hosted a major LGBT event.

Clinton, who was against gay marriage in her 2008 presidential run, spoke favorably about New York's gay marriage law when it passed in 2011, but has remained quiet on her position since, as other Cabinet members such as Arne Duncan, along with Vice President Joe Biden, have voiced support, and as marriage referenda passed in four states earlier this month.

But according to two sources, Clinton's aides have privately indicated to people that she will end up where her husband and daughter, Chelsea, have emerged on the issue - in favor of same-sex nuptials.

Her statement of support will probably come during an interview, presumably after she leaves the office of Secretary of State. You say you thought Hillary Clinton endorsed gay marriage already? She probably hasn't supported it yet because, as a source tells Politico, she feels "because of her role as the country's chief diplomat that it was appropriate for her to stay out of this."