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 : 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.
Krauthammer's email is lett...@ charleskrauthammer.com.