bonfire of the marriages

I just finished watching a fascinating discussion on "Meet the Press" about the DOMA and Prop 8 cases facing the Supreme Court.
Lots of vitriol.
Many people, my dad included, cannot get past the definition of marriage as between a man and a woman.  Honestly I  had a hard time with that too, even as the Mario Cuomo Liberal that I am.
 But then I picked it apart, and looked at at this way:
We in this nation (made up of states that need to agree on this because of common federal interests and laws) recognize a legal partnership called "marriage" that has a certain meaning, rights, and responsibilities, just as we have a legal entity called a "corporation".  Anyone can form a corporation, so why shouldn't any two people form a marriage?  It is a legal entity held together by a contract, notwithstanding the religious or cultural definition and context.
What is so difficult about this?