Who said it has to make sense?
You're talking electoral politics in the US here. Looking for sense is a fool's errand.
In short, you don't (and more specifically, Kerry didn't) have to fully explain it. What he needed to say was:
"Marriage is handled by the states, and it won't be my place as President of this country to tell them how they should handle it."
... and he should have gone on to say "by the way... take a look at your communities. Are they better off or worse off than they were four years ago? How are your schools holding up? Are they shortening the school year because they can't afford to keep them open?"
The Kerry campaign (and Democrats in general) should be looking at and issues that work in red states. The current concentration by both parties on swing states to the exclusion of non-swing states is going to do bad things to the national fabric.
