"Nationalized gay marriage, now inevitable."

Explains Charles Krauthammer, who's not happy about this.

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 [Equal Protection] rationale leads — nationalizing gay marriage.... 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 on DOMA with the court’s four liberals, take that step?

It's about timing, slowing things down a bit and giving people a chance to accommodate to things.