But the court didn’t invalidate the law on federalism grounds. Instead it invoked the due process clause of the Fifth Amendment, which prevents the federal government from depriving people of life, liberty or property “without due process of law,” and which has long been understood to contain an equal protection component.
Because the Defense of Marriage Act discriminates against same-sex couples for no legitimate reason, because its “purpose and practical effect” are to impose “a stigma upon all who enter into same-sex marriages,” and because it reflects (in the court’s view) “an improper animus or purpose,” it violates the Fifth Amendment.
As Sunstein notes, in addition to his concerns about human dignity, Kennedy also has concerns about the scope of federalism and the rights of states (he leans heavily towards states' rights vs. federalism). But his concern to safeguard human dignity, which flows directly from federal constitutional principles even though the word itself does not appear in the "Constitution," is clearly the centerpiece of his ruling.
Sunstein concludes,
There are also hard questions about the meaning and scope of the notion of dignity. That notion should be taken not as freestanding or as a recent invention, but as a modest and historically grounded effort to help specify what a denial of equal protection looks like. If the government is demeaning, humiliating and stigmatizing people, there is a constitutional problem. That problem is hardly disconnected from the concerns that long ago gave rise to the national repudiation of slavery and Jim Crow -- and to the enduring constitutional principles that reflect that repudiation.
As a matter of simple fact, American constitutional law has never been static; a disciplined dynamism is built into our constitutional fabric. That dynamism is most inspiring, most constrained and least dangerous when the court refuses to build out of whole cloth, but instead maintains continuity with the basic purposes of the constitutional design. In resting its ruling on the foundation of human dignity, the Windsor opinion does exactly that.
As the United States Catholic bishops emphasize, safeguarding human dignity has long been at the very center of Catholic social teaching. As the Catechism of the Catholic Church notes (¶ 1700, 1929-1938), the notion of human dignity is a foundational building block of Catholic social teaching because it's rooted in the belief that all human beings derive their dignity through God's creation of humanity, and fundamental human rights flow from this recognition.
It might well be argued that, in accenting human dignity in his ruling striking down DOMA, Justice Kennedy was simply echoing central emphases of Catholic teaching. But if we reach that conclusion, then we'd have to ask whether the U.S. Catholic bishops, under the leadership of Cardinal Dolan and Archbishop Cordileone, are faithfully representing Catholic social teaching through their attacks on the dignity of LGBT human beings, wouldn't we?