Conservatives feel that Strait up normal marriage is good for this country. The *traditional* family unit is what they want to build this country on.
They dislike anything that would remove any aspect of it. As it stands, Bush is heavily pushing marriage as something that stablizes one's life, which I actually agree with. But he's doing it as part of a faith-based thing. It's not just Marriage he's pushing but traditional conservative Christian Marriage.
So he might say (I think he would) that "Marriage" is the victim when "Marriage" is changed in any way. And that in turn hurts American values, America's future, etc.