But 31 states have amended their constitutions to prohibit same-sex marriage. North Carolina was the most recent example in May. In Minnesota earlier this month, voters defeated a proposal to enshrine a ban on gay marriage in that state's constitution.
The biggest issue the court could decide to confront comes in the dispute over California's Proposition 8, the constitutional ban on gay marriage that voters adopted in 2008 after the state Supreme Court ruled that gay Californians could marry.
The case could allow the justices to decide whether the U.S. Constitution's guarantee of equal protection means that the right to marriage cannot be limited to heterosexuals.
A decision in favor of gay marriage could set a national rule and overturn every state constitutional provision and law banning same-sex marriages. A ruling that upholds California's ban would be a setback for gay marriage proponents in the nation's largest state, although it would leave open the state-by-state effort to allow gays and lesbians to marry.
In striking down Proposition 8, the 9th U.S. Circuit Court of Appeals crafted a narrow ruling that said because gay Californians already had been given the right to marry, the state could not later take it away. The ruling studiously avoided any sweeping pronouncements.
But if the Supreme Court ends up reviewing the case, both sides agree that the larger constitutional issue would be on the table, although the justices would not necessarily have to rule on it.
Throughout U.S. history, the court has tried to avoid getting too far ahead of public opinion. The high court waited until 1967 to strike down laws against interracial marriage in the 16 states that still had them.
Some court observers argue that the same caution will prevail in the California case.
