The Supreme Court's decision on Hollingsworth v. Perry, the case dealing with California's Proposition 8, did not occur along ideological lines as the DOMA decision did. While DOMA was about whether a law was constitutional or not, the Prop 8 case was about whether the dispute over that law belonged at the Supreme Court in the first place.
A majority of justices — Roberts, Scalia, Ginsburg, Breyer, and Kagan — ruled it didn't. The logic is that in order to sue over a law, you must show the law has harmed you in a demonstrable way. A District Court in California ruled that Prop 8 was illegal, and if California had then appealed in order to defend its own law, it's pretty clear the state would have standing. But the state government declined to defend Prop 8, meaning a group of random conservative lawyers got together and decided to do it. The court ruled they did not have standing to challenge the District Court's decision.
The really interesting question is why the four dissenting justices — Kennedy, Alito, Thomas, and Sotomayor — thought the lawyers did have standing to appeal. The dissent by Justice Anthony Kennedy (who, incidentally, is a Californian) focuses on the unique nature of California's ballot-initiative system, which allows citizens to propose additions to the state constitution that can pass by a simple majority vote statewide (Real Talk: this system is a huge part of why the state is impossible to govern). Kennedy's dissent claims this system takes lawmaking power away from the state government and puts it in the hands of everyone in the state, which means that if you're a Californian, you have standing to sue over laws in California.
Here are some key quotes:
"Under California law, a [ballot initiative] proponent has the authority to appear in court and assert the State’s interest in defending an enacted initiative when the public officials charged with that duty refuse to do so. The State deems such an appearance essential to the integrity of its initiative process. ... The State Supreme Court’s definition of proponents’ powers is binding on this Court." (pg. 2)
"The Court concludes that proponents lack sufficient ties to the state government. It notes that they “are not elected,” “answer to no one,” and lack “‘a fiduciary obligation’” to the State. But what the Court deems deficiencies in the proponents’ connection to the State government, the State Supreme Court saw as essential qualifications to defend the initiative system. The very object of the initiative system is to establish a lawmaking process that does not depend upon state officials. In California, the popular initiative is necessary to implement 'the theory that all power of government ultimatelyresides in the people.'”
"A prime purpose of justiciability [granting standing to argue a case] is to ensure vigorous advocacy, yet the Court insists upon litigation conducted by state officials whose preference is to lose the case. ... And rather than honor the principle that justiciability exists to allow disputes of public policy to be resolved by the political process rather than the courts, here the Court refuses to allow a State’s authorized representatives to defend the outcome of a democratic election."
"In the end, what the Court fails to grasp or accept is the basic premise of the initiative process. And it is this. The essence of democracy is that the right to make law rests in the people and flows to the government, not the other way around. Freedom resides first in the people without need of a grant from government. The California initiative process embodies these principles and has done so for over a century. ... In California and the 26 other States that permit initiatives and popular referendums, the people have exercised their own inherent sovereign right to govern themselves. The Court today frustrates that choice by nullifying ... a State Supreme Court decision holding that state law authorizes an enacted initiative’s proponents to defend the law if and when the State’s usual legal advocates decline to do so."