The Supreme Court agreed Friday to rule on the constitutionality of same-sex marriage, a step it refused to take less than two years ago when it let stand a federal ruling allowing those unions in California but declined to make marriages between gay and lesbian couples legal throughout the land.

Now that 36 states and the District of Columbia, holding about three-fourths of the country’s population, permit same-sex marriages, legal experts believe the high court is ready to settle the matter once and for all.

Faced with a split in the appellate courts, the justices will hear 2½ hours of oral arguments in April and issue a ruling before the court term ends in June.

“We are poised for a very important decision that will settle once and for all one of the great civil rights questions of our time,” said Richard Socarides, a onetime senior adviser on gay issues to former President Bill Clinton at a time when Clinton felt a political need to sign the 1996 Defense of Marriage Act. That measure denied federal benefits to same-sex couples and allowed states to refuse to recognize such marriages from other states.

The court will take up the single appellate court ruling from the Sixth Circuit that ruled against lifting state bans on gay and lesbian marriages. The arguments will consolidate four cases from Michigan, Ohio, Kentucky and Tennessee. One of the plaintiffs is a couple married in California and living in Tennessee, said Kate Kendell, executive director of the San Francisco’s National Center for Lesbian Rights.

“Thankfully, the Supreme Court seems poised to weigh in on the ultimate question of whether it is offensive to cherished constitutional principles for certain couples to enjoy full protection and security for their relationships and … discriminate against couples who by an accident of geography do not have such recognition,” Kendell said.

Confidence ‘premature’

Same-sex marriage opponents also hailed the court’s decision to rule on the issue. Rob Schwarzwalder, senior vice president of the conservative Family Research Council, called the court’s move “unavoidable.”

“This is an issue that is extremely pressing socially and legally, particularly in light of the fact that 20 judges have overturned state laws and state referenda that have affirmed the traditional definition of marriage,” Schwarzwalder said.

He called the confidence of gay rights activists “premature” and said the court will have to decide if “a relative handful of judges on the federal bench have the constitutional or moral authority to overturn the will of the people.”

State bans on same-sex marriage became widespread after a Republican-led Congress passed the Defense of Marriage Act in 1996 and former President George W. Bush made gay marriage an issue in his 2004 re-election campaign. Even comparatively liberal California voters passed the Proposition 8 ban on same-sex marriages in 2008 after the courts had ruled such marriages legal.

But cultural norms had already begun a rapid shift that took even gay activists by surprise. By 2011, after waffling on his support for same-sex marriage, President Obama, joined by Attorney General Eric Holder, said they would no longer defend the Defense of Marriage Act.

Major rulings

In June 2013, Supreme Court Justice Anthony Kennedy’s landmark opinion in United States vs. Windsor struck down DOMA. At the same time, while refusing to find a constitutional right to marriage on equal protection grounds, the court let stand the federal court decision to overturn Prop. 8 in Hollingsworth vs. Perry. California immediately became the 13th state to allow same-sex marriage.

As dissenting Justice Antonin Scalia warned, the sweeping language of the Windsor decision opened the floodgates to court rulings lifting state marriage bans across the nation, even in the most conservative states.

The Supreme Court held back on taking up challenges to these rulings until finally the Sixth Circuit Court of Appeals, based in Ohio, became the first federal appellate court to uphold the state bans, creating a conflict in the lower circuits that warranted the high court’s intervention.

“If a federal court denies the people suffrage over an issue long thought to be within their power, they deserve an explanation,” Judge Jeffrey Sutton said in his opinion upholding the four state bans. “We, for our part, cannot find one.”

Forcing their hand

Socarides said: “They took the one case where a circuit court ruled against marriage. I think many of the justices were hoping that all the circuit courts would come down in favor of gay marriage and they might not have to rule, but they got tripped up by Judge Sutton in the Sixth Circuit, which forced their hand.”

Socarides said it is “almost inconceivable at this point, with 35 states allowing gays to marry, that they could rule any other way … and turn back the clock on civil rights at this point.”

Carolyn Lochhead is The San Francisco Chronicle’s Washington correspondent. E-mail: