Marriage equality for gays and lesbians took a giant step forward Tuesday when a three-judge panel of the U.S. Court of Appeals in California overturned a ballot initiative passed by voters in 2008 that had banned gay marriage.

Voters approved Proposition 8 that year in response to an earlier ruling by the California Supreme Court legalizing gay marriage. After the adoption of Proposition 8, a federal judge ruled that it violated the constitutional rights of same-sex couples, and the appeals court Tuesday upheld that ruling.

The next step in California will likely be an appeal to the full court; whoever ends up on the losing side there is likely to appeal to the U.S. Supreme Court.

The outcome in the Supreme Court is far from certain. The justices’ views of gay rights have been evolving, and much may hinge on the position of Justice Anthony Kennedy.

Kennedy is often viewed as the swing vote on the court, sometimes aligning himself with the conservative bloc and sometimes with the liberal. In 2003 he wrote the majority opinion in the famous Lawrence case rejecting a Texas law outlawing homosexual contact between adults. It was a significant case because the court was overturning a Supreme Court ruling from 1986 when it had upheld a Georgia anti-sodomy law.

In the Lawrence case Kennedy found that the state did not have a right to interfere in the private sexual lives of those who had been charged with violating the law. “The petitioners are entitled to respect for their private lives,” Kennedy wrote. “The state cannot demean their existence or control their destiny by making their private sexual conduct a crime.”

By upholding the right to sexual privacy, the court may have laid the foundation for a similar finding with regard to the decision of whom to marry. Marriage equality may face a higher hurdle, however, because opponents of gay marriage argue that marriage is about more than the intimate conduct of two people. They argue that it’s about the propagation of children.

And yet courts around the country that have upheld marriage equality have recognized that the right to choose a marriage partner, like the right to choose a sexual partner, is a fundamental right that may not be infringed by the state.

In an extraordinary paragraph in the Lawrence decision, Kennedy made a stirring case for the kind of judicial activism required to overturn precedents that violate constitutional rights. He said those who had written and adopted the Fifth and 14th amendments “knew times can blind us to certain truths and later generations can see that laws once thought necessary and proper in fact serve only to oppress.

As the Constitution endures, persons in every generation can invoke its principles in their own search for greater freedom.”

The California case is another instance when the will of a majority of voters has been found to run afoul of the Constitution. As in the great civil rights cases of the past, it often falls to the courts to uphold the rights of minorities. In fact, the Constitution’s protection of minority rights against the overweening power of the majority is one of the great glories of the Constitution.

Public opinion is shifting on gay marriage. A Supreme Court ruling overturning the constitutional provisions and laws of numerous states banning gay marriage would provoke an upheaval comparable to court rulings banning racial segregation. But increasingly, the public approves of marriage equality. Six states allow same-sex marriage, including one of the largest, New York (and one of the smallest, Vermont).

The upheavals after the broadening of constitutional rights generally fade as people learn that to widen the arena of freedom in our nation strengthens democracy for us all.