You have a number of sincerely conservative justices on the Supreme Court who could not rule in favor of gay marriage. Scalia, Alito and Thomas, for example, would never be on the supporting opinion. At the same time, all Circuit Court rulings to reach the Supreme Court has been the same decision. Overturning those decisions would be difficult and extremely unlikely. Roberts has already shown himself to be pragmatic enough not to be on the wrong side of history with the ACA, even though he ruled in favor of DOMA.
With that said, it would be likely that the SC would rule to uphold the Circuit Courts 5-4 or 6-3. And in doing so, it would put the conservative component of the Supreme Court in the position of writing a dissent against it. I don't think they want to do that, for historical reasons. Still, there are many ways you can parse the outcome from this non-decision.
On one side it makes gay marriage immediately legal in a number of states that had their appeals denied. It is now immediately legal in Utah, Indiana, Wisconsin, Oklahoma and Virginia. So it provides immediate relief to gays in those states. Had the court taken up the cases, they would not have had the right to marry until the court, hopefully, ruled in their favor sometime next year. Cases pending in the Circuit Courts that had been held in favor of waiting on the Supreme Court will now proceed. In Colorado, the Tenth District, the Circuit Court had stayed a lower court ruling overturning the laws there. They did this, even though they had decided in favor of overturning the laws in Utah and Oklahoma. With no appeals left on their own decision, it is likely they will quickly uphold and release the stay on the Colorado decision as it is already aligned with their previous decision. There are affirmative cases that can be upheld sitting at the Circuit Court levels in three more districts, including District Five (Louisiana, Mississippi and Texas), District Six (Kentucky, Michigan, Ohio and Tennessee) and District Nine (a number of western states where gay marriage is already legal, but also including Alaska, Arizona, Idaho, Montana and Nevada). If these cases proceed and the courts rule in favor of gay marriage, it is likely the courts will no longer stay their decisions pending an appeal to the Supreme Court. The other Circuit Courts would likely decide that since the Supreme Court had not heard appeals on the decisions from the Fourth, Tenth and Seventh Circuit Courts today they would probably not hear an appeal coming from their districts as well.
So in essence, the Supreme Court has likely mortally wounded, but not killed, gay marriage laws in the United States.
But in some sense it's a cowardly decision because it will take more time for each state to process it's own judicial action to overturn all the gay marriage bans. The Supreme Court, by not taking on any of the cases today, has stated to all the lower cases that they would not overturn their decisions. Already over 50% of Americans live in states with legalized gay marriage. The Supreme Court will not change its mind next year when that will be an even larger percentage. So the Supreme Court is ready to affirm the Circuit Court decisions now. Yet the still chose not to. While it permits gay couples to get married immediately in five states, it denies hope to gay couples in other states for immediate relief next year. They will have to wait even longer. That is justice delayed.