Defining "marriage", in federal law, as only a heterosexual union opens a Pandora's box of later defining it as only applying to Europeans, or whites, or blacks, or Asians -- or people who enter the union in city hall (to go with need for a state granted license).
Obviously the state does have the right to define who is granted the right to file as married under federal or state tax codes; it also has the right to define who can be counted as a survivor or have a defacto power of attorney. But, the state also has the obligation to show clear damage to the social fabric when it excludes individuals from benefiting from rights granted to society as a whole. As it stands, homophobic politics by bigoted sudo-religious groups, who misrepresent scripture, has already poisoned American politics. The court can allow that poisoning to continue, or, show that the government represents the right of all people to enter into committed relationships.
Those who doubt the poisoning, I would point out that since the "defense of marriage" group defined marriage in terms of having children, the marriage rates have dropped. If you are not going to have children, the logic goes, there is no justification for marriage -- love and the desire to spend your life with someone, especially in the absence of any intent to have biological children, is insufficient. That is, after all, the official logic behind the defense of marriage. Defense of marriage has poisoned marriage by attacking those who want to marry.