SMore than 100 briefs that have been filed on both sides arging that U.S. v. Windsor should prompt a decision in their favor in the marriage case the court will hear this spring. (Tamir Kalifa/AP)

It would seem indisputable that the people who know better than anyone else what the Supreme Court was saying in its landmark decision on same-sex marriage are the nine justices themselves.

After all, the 5-to-4 ruling in U.S. v. Windsor that struck down a key portion of the federal Defense of Marriage Act came less than two years ago. It was the most scrutinized and consequential case in a term of big decisions. The court’s most influential member, Justice Anthony M. Kennedy, wrote the majority opinion, and three of the dissenters wrote to explicate their own understandings of the ruling.

But as the court prepares for its most important case on gay marriage — whether state prohibitions on same-sex marriage are unconstitutional — the justices and their clerks are plowing through a mountain of briefs that confidently, and contradictorily, instruct the court on what the Windsor decision meant.

It’s a bit of a strange and “delicate enterprise” for the attorneys, said Kannon Shanmugam, a Washington lawyer and frequent advocate at the Supreme Court who is not involved in the same-sex marriage case.

“The Supreme Court has cases all the time” whose outcomes would seem to hinge on the interpretation of a single precedent, he said. “But it’s rare to have one that is so recent and so high-profile. . . . The justices can hardly have forgotten” what they meant.

Nonetheless, most of the more than 100 briefs that have been filed on both sides argue that the Windsor decision demands a decision in their favor when the court decides whether states may limit marriage to a man and a woman and refuse to recognize same-sex marriages performed elsewhere.

“Like the federal law invalidated in Windsor, Kentucky’s marriage ban is borne of moral disapproval of couples ‘whose moral and sexual choices the Constitution protects,’ ” says a brief from couples challenging that state’s ban on gay marriages and the recognition of those conducted where it is legal.

“It makes no difference that Windsor involved federal recognition of a state marriage, whereas this case involves Kentucky’s recognition,” that brief says. “Whether recognition is denied by state or federal government, the impingement on equal dignity for individuals is every bit as severe.”

But Michigan, defending its decision to limit marriage to heterosexuals, said Windsor is supportive of its position.

Michigan’s brief notes the acknowledgment in Kennedy’s opinion that “until recent years, many citizens had not even considered the possibility that two persons of the same sex might” marry. So, the state asks, how can the Constitution command it?

“To recognize a right to marriage without gender [distinction], the court would have to abandon the reasoning and holding of the decision it issued just two years ago in Windsor,” the brief says.

Kyle Duncan, a Washington lawyer who filed an amicus brief on behalf of Louisiana and other states that want to retain their same-sex marriage prohibitions, devotes a long section of his brief to the Windsor decision.

“Yes, it is a little bit curious telling the Supreme Court, ‘This is what you decided,’ ” Duncan acknowledged. The court’s precedents are often from years gone by instead of fresh and controversial.

“I think the rapidity of this returning to the court has caught everyone off guard a little bit,” he said.

A federal judge agreed with Duncan in the lower court and upheld Louisiana’s law. But that ruling was an outlier.

More than five dozen court decisions since the Windsor decision have gone in favor of gay-marriage advocates. Federal appeals courts in Denver, Chicago, Richmond and San Francisco, covering a broad swath of the nation, have struck down state prohibitions on same-sex marriage, and it is now legal in 37 states and the District of Columbia.

But an appeals court in Cincinnati upheld the prohibitions in Michigan, Ohio, Kentucky and Tennessee, giving rise to a split in the nation’s appeals courts that has compelled the Supreme Court to step back in.

The court’s decision in Windsor struck down a law that withheld federal recognition of same-sex marriages performed where they are legal. All nine justices agreed that the ruling did not settle the question of whether states must allow such unions, as advocates say the Constitution’s guarantee of due process demands.

Kennedy’s opinion, joined by the court’s liberals, gave ammunition to both sides. Half of his opinion rests of the traditional respect the court has shown for states to set their own laws regarding marriage. The other used soaring language to describe a state’s decision to allow gay couples to marry, saying it “enhanced the recognition, dignity and protection of the class in their own community.”

State sovereignty must account for constitutional guarantees, Kennedy wrote.

Justice Antonin Scalia, in his much-quoted dissent, said that the majority had armed gay rights advocates to go after state prohibitions, and he has been proven right. The Supreme Court has repeatedly decided since then not to put on hold the rulings of lower courts forcing reluctant states to allow same-sex marriage, and this has strengthened the view that a majority of the court has made up its mind.

Irv Gornstein, a longtime Supreme Court practitioner who is now director of the Supreme Court Institute at Georgetown Law Center, said only the Supreme Court knows exactly what Windsor meant, and advocates are unlikely to sway the justices on that.

“There are some good lines in there that I would quote” if he were writing a brief for either side, Gornstein said. “I just wouldn’t try to pretend it decided the question either way.”