DOMA cannot survive under these principles. Its unusual deviation from the tradition of recognizing and accepting state definitions of marriage operates to deprive same-sex couples of the benefits and responsibilities that come with federal recognition of their marriages. This is strong evidence of a law having the purpose and effect of disapproval of a class recognized and protected by state law. DOMA’s avowed purpose and practical effect are to impose a disadvantage, a separate status, and so a stigma upon all who enter into same-sex marriages made lawful by the unquestioned authority of the States.
DOMA’s history of enactment and its own text demonstrate that interference with the equal dignity of same-sex marriages, conferred by the States in the exercise of their sovereign power, was more than an incidental effect of the federal statute. It was its essence.
Read the decision, here.