"It is not enough that the party invoking the power of the court have a keen interest in the issue," Chief Justice John Roberts Jr. wrote, adding that "we have no authority to decide this case on the merits."
The decision eliminates a 9th U.S. Circuit Court of Appeals ruling and leaves intact a trial judge's order blocking Proposition 8 from taking effect. Typically, it takes about 25 days for a Supreme Court ruling to filter down to the lower courts, though advocates hope it will happen sooner.
"We're elated," Berkeley, Calif., resident Kris Perry -- one of the individuals who challenged Proposition 8 -- said at a news conference. "Now our children will finally be in a family where their parents are married."
Section 3 of the Defense of Marriage Act declares that, for the purposes of providing federal benefits, marriage is "only a legal union between one man and one woman as husband and wife" and a spouse is only a "person of the opposite sex who is a husband or a wife."
The definition is important because it determines eligibility for a host of federal rights, benefits and privileges.
The Government Accountability Office has identified more than 1,100 areas of federal law in which marriage matters, ranging from tax and welfare benefits to employment and immigration. Same-sex military couples, for instance, are denied housing, health insurance and disability benefits, and are ineligible for burial alongside their spouses in national cemeteries.
"The statute is invalid, for no legitimate purpose overcomes the purpose and effect to disparage and injure those whom the state, through its marriage laws, sought to protect in personhood and dignity," Kennedy wrote.
Twelve states -- 13, including California -- and the District of Columbia recognize same-sex marriage, and a recent Pew Research Center survey found that 72 percent of those asked thought that legal recognition of gay marriage was inevitable.