Good -- and, as Sebastian notes, bad, too, because they went and invented some more specialness to do it -- the Supremes have, at least, kinda decided the Feds haven't got any excuse to go regulating marriage.
IMO, any time government gets told "hands off!" that's a good thing.
It is also (and also IMO) a good thing if this drives a wedge between the religious ceremony/bond of marriage and the civil contract called marriage. They're not the same. Americans are a bit axle-wrapped about it, as we inherited the institution from a nation with a State Church; it wasn't much noticed until some states of the union decided they'd better keep couples of differing "races" from marrying, and while they were at it, people too closely related -- and, in many U. S. states, "imbeciles," too.
Is that a legitimate interest of the State, or of a state? I'm not seeing it. Churches, sure, religions have all kinds of rules about who can, can't and must get wedlocked, and they don't all jibe with one another, either, which is fine, since (at least in the U.S.) their authority extends only to the adherents of their religion.
The usual worrywarts have come out of the woodwork, down from their pulpits and out from under rocks, asking--
"What's to stop cousins and siblings from marrying, then?" --Hell, I dunno. "Decorum and good taste" is probably out and if they lack socially-normal ingrained abhorrence of inbreeding,* they probably are reinforcing recessives already. So the question is really about your right to not be squicked by social deviates; you're fine if they've got to sneak around.
"What's to stop polygamous marriages? Group marriages?" Bloody-minded prejudice, if you asked the LDS at the right point in history, and that probably motivated by fear of being out-earned and out-bred. Also, these days we have plenty of TV shows demonstrating the pros and cons of one version of polygamy -- I wouldn't sign up for it on a bet. But there's no rational basis at all to limit wedlock to only two people. And we've got lots of divorce lawyers who'd welcome the work. (Man, they'd get fat on it!)
"How about an adult marrying a minor or horse?" (or "...a chair?" etc.) Aha! That, there's a basis to deny: none of those entities can give meaningful consent or enter into a binding contract.
Somehow, when the Framers, Federalists, anti-Federalists and worried states adopted the First Amendment and the courts applied it, they overlooked marriage. It's well-past time to separate Church weddings and State-licensed marriage. Let the religions have their lovely ceremonies and oaths, and let the civil contract stand apart. Participants pick one or the other or both.
...Just think, I haven't even mentioned things like inheritance, income tax (you got your marriage, now you get the IRS marriage penalty: pay up, L/G/B suckers!), the mutual power of attorney and protection in court spouses get that non-officially-espoused partners do not. Sure, homosexual couples can sort of get most (but not all) of that now with the help of an attorney, a lot of spare time and a few thousand bucks; but in the crunch, when one partner is in a coma or whatever, it's just a license to sue, not the nearly-sure thing the legally-married get.
Now the ongoing debate goes to the states. Gonna be interesting for awhile. Gonna be real interesting and probably nobody will like the compromises that spring up. The base on both sides will be all fired up and the papers and broadcasters will get lots of copy and images. --Might even get louder than the gun debate, or at least be more of a debate; there's less astroturf, I think.
(For the few who will continue to maintain It Is Just A Conspiracy By The Gays To Ruin Christianity? Sure; sure it is, you are so very right, just the same way the Jews run the banks and media, Freemasons run the world, all African-American people are stupid, all Republicans are child-labor-exploiting plutocrats, all Democrats are bloody-handed Stalinists, all Asians are sneaky math geniuses, women conspire to hold men subservient, the Patrarchy keeps womyn down and the Moon landings were filmed in the same vast soundstage where they faked the Zapruder film. Look up "prejudice." Look up "blood libel," and stop spouting that crap while telling me how some of your very bestest and most dearest pals are queer, Jewish, Masons, black, party-line voters, Asian, women, men, rocket scientists and small-time sportswear magnates who owned 8mm film cameras. You make me want to puke.)
* Which puts them right up there with some New World Royalty. Old World, too, depending on how close cousins you count. The Hapsburgs and Romanovs clearly didn't count all that well. And didn't the ancient Egyptians...? Historically, it hasn't worked well, though, tragically, failure happens over a long timebase until the unlucky winner comes into the world daft or chronically ill through no fault of his or her own -- but what if they gene-tested everyone, and refused marriage to any breeding pairs if the odds were bad enough for their offspring? Would that be okay, or an overreach?
STANCOR 10P TRANSMITTER: UPDATE 12
3 hours ago