There is a difference between believing something and forcing others to act according to your beliefs.
Kerry's statement [in response to Bush finally saying that he supports a constitutional ammendment defining marriage as between one man and one woman] says things like
"While I believe marriage is between a man and a woman, for 200 years, this has been a state issue"
and
"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"
That is, Kerry is saying that personally he believes one thing but he's not going to support something to force everyone to act according to his personal beliefs.
This I find reasonable. Attitudes like this allow people with different ideas (i.e. people) to live with each other.
And yes, I understand that a relaxed, tolerant attitude can be taken too far. You can't say "Yeah, I'd prefer that people don't murder each other or me but I'm gonna let them decide for themsevles."
What's the difference? Letting people murder hurts someone; letting consenting adults - no matter what sex they are - marry hurts no one.
Furthermore allowing people to murder destroys society; allowing consenting adults - no matter what sex they are - to marry builds society.
I don't agree with Kerry that marriage should be between a man and a woman but I do support the fact that he is opposed to this attempt to force this belief on others.