In a 5-4 opinion, authored by Justice Kennedy, the Supreme Court declares Section 3 of DOMA unconstitutional. It’s an Equal Protection holding, informed by federalism principles. Here is how Justice Kennedy’s opinion for the Court concludes:
The power the Constitution grants it also restrains. And though Congress has great authority to design laws to fit its own conception of sound national policy, it cannot deny the liberty protected by the Due Process Clause of the Fifth Amendment.
What has been explained to this point should more than suffice to establish that the principal purpose and the necessary effect of this law are to demean those persons who are in a lawful same-sex marriage. This requires the Court to hold, as it now does, that DOMA is unconstitutional as a deprivation of the liberty of the person protected by the Fifth Amendment of the Constitution.
The liberty protected by the Fifth Amendment’s Due Process Clause contains within it the prohibition against denying to any person the equal protection of the laws. See Bolling, 347 U. S., at 499–500; Adarand Constructors, Inc. v. Peña, 515 U. S. 200, 217–218 (1995). While the Fifth Amendment itself withdraws from Government the power to degrade or demean in the way this law does, the equal protection guarantee of the Fourteenth Amendment makes that Fifth Amendment right all the more specific and all the better understood and preserved.
The class to which DOMA directs its restrictions and restraints are those persons who are joined in same-sex marriages made lawful by the State. DOMA singles out a class of persons deemed by a State entitled to recognition and protection to enhance their own liberty. It imposes a disability on the class by refusing to acknowledge a status the State finds to be dignified and proper. DOMA instructs all federal officials, and indeed all persons with whom same-sex couples interact, including their own children, that their marriage is less worthy than the marriages of others. The federal statute is invalid, for no legitimate purpose overcomes the purpose and effect to disparage and to injure those whom the State, by its marriage laws, sought to protect in personhood and dignity. By seeking to displace this protection and treating those persons as living in marriages less respected than others, the federal statute is in violation of the Fifth Amendment. This opinion and its holding are confined to those lawful marriages.
The Chief Justice, Justice Scalia, and Justice Alito all author dissents. The Chief Justice emphasizes the federalism aspects of Kennedy’s majority opinion. Justice Scalia (joined by Justice Thomas and the Chief on this point) would have held that the Court lacked jurisdiction. Justice Alito, in dissent, notes that the jurisdictional question was closer in Windsor than in Perry, so it is somewhat incongruous to find jurisdiction to invalidate DOMA while holding there is no jurisdiction to review Proposition 8. All four dissenters would have upheld DOMA, though the Chief Justice, Justice Scalia and Justice Alito offer slightly different rationales.