in their words there's more to it than a piece of paper...
I can't speak for everyone, but I can speak for me.
I want to get married because I am in love and I want to make a life long commitment to my love. I want to stand up in front of friends and family and way "we are together forever."
I want to _be_ married, legally, so that if anything happens to my girl I am legally allowed to take care of her, make medical decisions for her. I want the more then 1000 rights and responsibilities that marriage grants.
But I can get a civil partnership, right? Sure, but it's only valid in California. If I was visiting my girlfriend's parents in Florida and there was an accident they could keep me out of her hospital room. She could be dying and they could legally prevent me from saying goodbye. They can't do that if you are married.
I want my kids to have two legal parents. And if something happens to either one of us I want to know that my kids are protected, not in danger of being taken away from their parent.
I want to be equal to everyone else in the country, because I am.
and
There are legal consequences to marriage that are not available to non-married people, no matter how close and committed they may be.
For example, in the absence of a will, property from one married partner automatically transfers to the surviving spouse.
Another is health care coverage. Not all health insurance plans explicitly cover "domestic partners" (although many do).
The list of such benefits is fairly lengthy.
While it is true that most of these benefits can be contractually arranged, not all of them can (e.g. tax benefits) and it would be immensely easier for the same-sex couple to just be legally married so that all of those benefits are automatically granted.
My prediction is that, eventually, we will have something called 'civil partnership' or 'domestic partnership' wherein two people can be recognized as "married" for legal purposes, but we won't call it "marriage" so as not to offend people who think God cares who marries whom.