In what will be a landmark decision, the U.S. Supreme Court agreed to hear four cases challenging same-sex marriage bans in four states on Friday. Their ruling, which is expected by June, would mark the culmination of a decade-long civil rights struggle that seeks marriage equality for millions of gay and lesbian Americans.

The justices granted certiorari to four couples, one each from Ohio, Tennessee, Michigan, and Kentucky, challenging those states' same-sex marriage bans. In November, a Sixth Circuit Court of Appeals panel ruled agains the four couples and upheld the bans. That decision created a circuit split among the federal judiciary, a schism of constitutional interpretation that the Supreme Court alone can (and often does) repair.

The court's decision comes two years after two landmark gay-rights rulings that set the stage for these cases. In 2013, the court ruled in U.S. v. Windsor struck down part of the Defense of Marriage Act and ruled that the federal government could deny equal rights to same-sex couples who lawfully married in states where it was legal. That same day, the justices vacated a Ninth Circuit ruling on California's same-sex marriage ban in Hollingsworth v. Perry, also known as Proposition 8, because those defending the ban lacked standing to appeal. Their decision left a lower court ruling that had overturned the ban, thereby allowing gay and lesbian couples in California to marry.

In a scathing dissent in Windsor, Justice Antonin Scalia predicted the ruling would use the court's reasoning to invalidate state bans on same-sex marriage—and so they did. Now, 36 states allow gay and lesbian couples to wed, many as a result of federal rulings following Windsor and Hollingsworth. Some federal judges even cited Scalia's prophesy in their rulings.

The court's order also announced it will decide the case on two questions: First, does the Fourteenth Amendment require a state to license a marriage between two people of the same sex? Second, does the Fourteenth Amendment require a state to recognize a marriage between two people of the same sex when their marriage was lawfully licensed and performed out-of-state?

This framing presents Justice Anthony Kennedy, the court's swing vote and the author of many of the court's gay-rights opinions over the last two decades, with a dilemma. The Fourteenth Amendment is an expansive one, and the framing gives the justices flexibility to decide the case under either the amendment's Equal Protection Clause or its Due Process Clause. Invoking the former to strike down the marriage bans would define sexual orientation as a protected class, thereby providing gay and lesbian Americans with greater constitutional protection from discrimination than they currently enjoy. Wielding the Due Process Clause, as Kennedy did when the court struck down sodomy laws in 2003's Lawrence v. Texas, would produce a far narrower precedent for future gay-rights cases by comparison.

Another wild card is Chief Justice John Roberts. Although a member of the court's conservative wing, Roberts also authored the court's opinion in Hollingsworth v. Perry that led to Proposition 8's demise. The case's technical nature meant that the chief justice avoided ruling on the broader Fourteenth Amendment issues then, but some observers speculate he could be a possible fifth or sixth vote for marriage equality.

Nothing is certain until it happens, and the court could still surprise the country with its ruling in June. But with the near-unanimous embrace of marriage equality by the lower courts and the social trajectory of gay rights in recent years, a Supreme Court ruling against marriage equality would likely last only as long as its authors.