Because a great many folks who can marry are choosing not to. Millions of them.
My own niece has 3 kids, a house and a good job. She was raised Catholic by parents in a solid marriage. And yet, she and her partner have never bothered to marry.
And those who ARE married seem to have less than sacred views of those vows.
Especially those on the right side of the political spectrum who howl about it the loudest. Folks like the ever faithful Senator Ensign and Governor Sanford.
Gay couples get it. They understand that marriage means legal security for your kids and for your spouse.
That it says "we live together and we're committed to doing so long term--hopefully forever".
What we really need is civil unions for all. And then the churches would be free to practice their own personal prejudices. We've got separation of church and state--remember?
But this country doesn't want civil unions for all. It HAS to be marriage.
So let gays marry. Let them raise kids in stable homes, pay taxes together. Do all the things they've done forever, just now with that legal stamp on their union.
And perhaps, when marriage is a right offered to all, then it will reclaim its value and its dignity.