Right now it would just be too much of a politically risky stance for any candidate running for national office to come out in favor of gay marriages.
Al Sharpton's the only Democratic candidate to have done it, but he's not under any delusions of winning. He's only running to make his voice heard and speak out on the issues so he can afford to take risky positions. Which is why it's so fun having him in the race though he never gets more than a percentage point in any of the caucases or primaries.

I say let Kerry and Edwards call for civil unions until they're blue in the face. Either way, an investment in their presidencies is an investment in gay marriage eventually. Because they, as presidents, could use the bully pulpit for greater tolerance of homosexuals and an increased acceptance of the idea of gay marriages among the general population, they would nominate more liberal judges to the Supreme and other federal courts, they would speak out against things like Constitutional amendments and the like.

There's no way in heck that the current move to amend the Constitution to ban gay marriages will ever climax before November. It needs to be passed by both the Senate and House and then sent to all 50 states for ratification. That'll be a long process. So even though Kerry and Edwards don't openly support gay marriage right now, either of them in the White House when a proposed amendment goes to the Senate and House floors and to the states if necessary - along with the message sent to conservatives that their incumbent president was defeated after submitting such an amendment to the Constitution - is vital.

Don't worry about the statements of Kerry and Edwards right now. Whichever one wins the Dem. nomination needs our full support with nothing held back.
And I'm a registered Republican!