As Atrios notes, the headline of this AP story -- "Kerry Signed Letter Backing Gay Marriage" -- is an utter distortion of the truth. Rightly or wrongly, Kerry's support for gay rights stops at the altar -- he supports civil unions. The proposed state constitutional amendment he objected to in the letter would have banned civil unions as well as gay marriage.
The text of the amendment is here and (in fuller form) here:
It being the public policy of this Commonwealth to protect the unique relationship of marriage in order to promote among other goals, the stability and welfare of society and the best interests of children, only the union of one man and one woman shall be valid or recognized as a marriage in Massachusetts. Any other relationship shall not be recognized as a marriage or its legal equivalent.
So when Kerry signs a letter objecting to a "proposed Constitutional amendment that would prohibit or seriously inhibit any legal recognition whatsoever of same-sex relationships," because of "the likelihood that it will prevent not only the state government, but also the cities, towns and counties from acting as they might wish to provide some form of recognition for same-sex relationships," and he later says he's never backed full gay marriage, there's no contradiction.
But Closet Case Drudge is trumpeting the story, so expect lazy superstar "journalists" to begin telling you Kerry's a flip-flopper and a liar.