Rich will have an in-depth analysis of the Supreme Court's gay marriage decisions later today. In the meantime, these two sentences, from Anthony Kennedy's opinion striking down DOMA, sum up the majority's argument pretty well:
The history of DOMA’s enactment and its own text demonstrate that interference with the equal dignity of same-sex marriages, a dignity 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.
At its core, the opinion is a states' rights argument, since it notes the tradition of the states regulating domestic arrangements in America and vests that power exclusively with the states. But Kennedy is also affirming the overarching principle of gay marriage by defending its "dignity" as an expression of equality, barring the federal government from interfering with state gay marriages by citing the Fifth Amendment's guarantee of due process under the law. Put shortly: equality for gay couples, decided and enforced by the states.
It's an argument that fits neatly with Kennedy's legal philosophy, which is most accurately classified as sweeping libertarianism with a healthy respect for federalism. With Kennedy as the Court's undisputed power broker, we may be looking at more libertarian-leaning decisions in the years to come.