WASHINGTON – Under the Defense of Marriage Act, the federal government does not recognize same-sex marriages even in states that have legalized it. This week, the Supreme Court ruled DOMA unconstitutional.

There are two possible grounds, distinct and in some ways contradictory, for doing so. The curious thing about the court’s DOMA decision is that it contains both rationales.

The first is federalism. Marriage is the province of the states. Each state decides who is married and who is not. The federal government may not intrude. It must therefore recognize gay marriage where it has been legalized.

If that were the essence of the argument, the court’s 5-4 decision would have been constitutionally conservative, neither nationalizing nor delegitimizing gay marriage. It would allow the issue to evolve over time as the people decide state by state.

It would thus be the antithesis of Roe v. Wade. That judicial fiat swept away every state abortion law that did not conform to the court’s idea of what abortion law should be. Even many liberal supporters of abortion rights have admitted that Roe was an unfortunate way to change the law. It prevented a stable social settlement of an issue, Justice Ruth Bader Ginsburg once said, that at the time was headed in the reform direction. The Roe ruling removed abortion from the political arena, thus disenfranchising the citizenry, tainting the resolution of the question and leaving us with 40 years of social strife.

On the face of it, the court avoided that disaster regarding same-sex marriage by adopting judicial modesty. Gay marriage? You, Washington, have no business meddling in state business. To those married and living where such marriage is recognized, you must provide the normal federal spousal benefits, etc. Otherwise, you don’t.

Good outcome. It allows popular sentiments on gay marriage to translate themselves democratically into law. Which in turn allows, in contradistinction to abortion, a political settlement of the question state by state. It may not satisfy everyone, but it does give everyone a voice in the disposition of the issue and a sense of the legitimacy of the outcome.

Except that in the DOMA decision, the court added a second rationale: equal protection. In states with same-sex marriage, Washington must give the same federal benefits to gay couples as to straight couples because to do otherwise is to discriminate against the gay couples. After all, they are equally married in their states. For Washington to discriminate against them is to deny them equal protection of the laws. Such discrimination is nothing more than irrational animus – and therefore constitutionally inadmissible.

But notice what that second rationale does. If the argument is just federalism, the court is saying: Each state decides – and we, the court, are out of here. But if the argument is equal protection, one question is left hanging. Why should equal protection apply only in states that recognize gay marriage? Why doesn’t it apply equally – indeed, even perhaps more forcefully – to gays who want to marry in states that refuse to marry them?

If discriminating (regarding federal benefits) between a gay couple and a straight couple is prohibited in New York where gay marriage is legal – by what logic is discrimination permitted in Texas where a gay couple is prevented from marrying in the first place?

Which is exactly where the majority’s second rationale leads – nationalizing gay marriage, the way Roe nationalized abortion. This is certainly why David Boies, the lead attorney in the companion Proposition 8 case, was so jubilant when he came out onto the courthouse steps after the ruling. He understood immediately that once the court finds it unconstitutional to discriminate between gay and straight couples, nationalizing gay marriage is just one step away.

So why didn’t Justice Anthony Kennedy, the traditional swing vote who wrote the majority opinion with the court’s four liberals, take that step? Why did he avoid doing the full Roe – nationalizing the procedure in question and declaring the subject now closed? I suspect he thought it would be a bridge too far. At least for today.

But he knows that the double rationale underlying his DOMA opinion has planted the seed for going Roe next time. It was prudence, not logic, that stayed his hand. “The only thing that will ‘confine’ the court’s holding,” wrote dissenting Justice Antonin Scalia with a bit less delicacy, “is its sense of what it can get away with.” Next case – Kennedy & Co. go all the way.

© 2013, The Washington Post Writers Group