Here is a nice concise summary of the case for marriage from Maggie Gallagher at the National Organization for Marriage.
There’s more reasoning in those two short pages than in a majority of the Washington State legislature.
And a lengthier article on the same subject by Jennifer Roback Morse.
Marriage is a naturally occurring, pre-political institution that emerges spontaneously from society. “Marriage equality” is sometimes presented as an enlargement of personal liberty and a diminishment of the power of the state. In fact however, redefining marriage as the union of any two persons, abandons the natural view of marriage. The institution of natural marriage, one man and one woman, is more consistent with a society of free and responsible individuals, governed by a constitutionally limited state, than the alternatives.
Both via the NOM Blog.