Several smart ones - such as Cuomo and Jay Inslee here in Washington - have figured out that publicly supporting it is a wise move.
Word on the street is that Cuomo and Inslee also realized that they are from New York and Washington respectively and this really isn't toxic at all in those states.
I should clarify - that comment is directed at the numerous Democrats who being in favor of marriage equality is now more political expedient than being opposed but haven't figured it out because of outdated thinking. Pretty much any Democrat in a state or district won by John Kerry should be in favor of marriage equality at this point.
There are of course, as you snidely imply, a number of Democrats where the political backlash would still be harmful. Though that number is rapidly shrinking.