Well, I got my answer. Kerry issued a statement saying he would vote against the President's proposed amendment. He cites a states'-rights argument in order to sort of dodge the bullet:
While I believe marriage is between a man and a woman, for 200 years, this has been a state issue. I oppose this election year effort to amend the Constitution in an area that each state can adequately address, and I will vote against such an amendment if it comes to the Senate floor.
I believe the best way to protect gays and lesbians is through civil unions. I believe the issue of marriage should be left to the states, and that the President of the United States should be addressing the central challenges where he has failed - jobs, health care, and our leadership in the world rather than once again seeking to drive a wedge by toying with the United States Constitution for political purposes.
His argument (continued elsewhere) is that gay civil unions should be given all the protection and benefits of marriage except the right to use the word "married" to refer to their relationship. As far as government is concerned, his position is completely in favor of gay marriage, but he keeps stressing this point that they should be called civil-unions and not marriage. I think it's interesting to see how powerful that single word is in our culture.