Amendment XIV
Section 1.
All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside. No state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any state deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws.
http://www.law.cornell.edu/constitution/amendmentxiv
No person shall be denied the equal protection of the laws.
The law protects the right to marry. In my view, no person should be denied the protection of marriage based on gender. If the law protects the right of a man and woman to marry, it should also protect the right of a man and a man or a woman and a woman to marry. To deny a man the right to marry just because he wants to marry man would be to deny that man the protection of marriage.
The status of marriage gives us many rights, and the first is the right to be recognized by the law as married to our spouse. In some courts, the fact that you are married to someone may affect whether that person may refuse to testify against you. There may be obvious tax advantages. A spouse may have pension rights that a mere partner might not have. There are a lot of legal rights that come with marriage.
That is my theory. I don't know whether it is the theory being used in the courts. But I think it should be.