What now for California? In May, its Supreme Court announced a right to same-sex marriage. Gays and lesbians rushed to take advantage of the opportunity; by early November, 18,000 such marriages had been performed. But on Nov. 5, they stopped. By a 52-47 percent margin, California voters approved Proposition 8, an amendment to the state constitution prohibiting same-sex marriage.

Immediately, gay rights supporters filed lawsuits asking to overturn the ruling. Critics are calling Proposition 8 an illegal constitutional "revision," fundamentally altering the guarantee of equality – not a more limited "amendment."

This suit raises a serious question: When should a majority have the power to take away a constitutional right granted by a court?

It's a question that forces us to think about why we have constitutional rights in the first place, and why they are enforced by judges. But it is not simply a theoretical puzzle. All of us enjoy constitutional rights, and most of us are at some point in a minority. All of us could be affected.

American constitutional practice has generally been to expand rights over time, both by amendment and by judicial decision. Amendments to the federal Constitution, for example, gave women and minorities the right to vote. Judicial decisions have expanded the constitutional guarantee of equality to protect more and more groups. Some of these decisions remain intensely controversial, but none have been overruled by a federal amendment.

Of course, amending the federal Constitution is difficult. It requires approval by "supermajorities": two-thirds in the House and the Senate and three-quarters of state legislatures. Federal rights cannot be taken away by a simple majority vote.

Because of this requirement, judicial decisions enforcing the federal Constitution's equality guarantee have followed a relatively consistent pattern. At one point in time, a particular practice – say, the racial segregation of public schools or the exclusion of women from the practice of law – is so widely accepted that it seems beyond challenge. Judges are not likely to strike the practice down, and if they did, the backlash might well be strong enough to create a constitutional amendment.

Some time later, the practice becomes controversial. It still enjoys majority support – otherwise it would likely be undone through ordinary lawmaking – but it no longer has the allegiance of a supermajority. It is at this time that judges tend to act in order to protect the freedoms of the minority, striking down the practice as unjustified discrimination. The decision may be intensely controversial. It may even be the target of majority disapproval. But because there is no longer a supermajority, the decision is safe.

As attitudes evolve, the practice comes to seem outrageous. Almost no one, nowadays, would argue for racial segregation of schools or a ban on female lawyers. At this point, the judicial decision is no longer controversial.

If a majority could overrule a judicial decision, the process would frequently be stopped by that majority vote. Judicial interventions against discrimination would just not succeed.

Regardless of where you stand on same-sex marriage, what's troubling for US citizens in the California case is the idea that an equality guarantee could not be effectively enforced against the will of a majority. The point of such a guarantee is precisely to protect minorities from discrimination at the hands of a majority.

It would be somewhat surprising, then, if California allowed judicial decisions enforcing the state equality guarantee to be overruled by a simple majority vote. In fact, as the gay-rights supporters' suit indicates, it is not clear that it does. Under the California constitution, "amendments" can be approved by a simple majority vote.

But "revisions," which make substantial changes, require approval by a supermajority – two-thirds of both houses of the legislature – before being submitted to voters. Supporters framed the same-sex marriage ban as an amendment, when really it has the makings of a revision.

It makes sense to require supermajority support to overrule a judicial decision that grants rights to a minority. It shows that the judges were so out of step with society that they were probably wrong. But a simple majority does not show that, and the constitution would not afford meaningful protection if it could be overruled at the will of the majority.

As the opposition to same-sex marriage in California has shrunk, simple majorities should not be able to reverse decisions made in the name of equality.

This is not an argument that the California court was correct. The battle for public opinion goes on. But letting the court's decision stand against the disapproval of a simple majority is not only sensible, it protects the minority rights of future generations.

Unpopular decisions are the price of constitutional rights.

• Kermit Roosevelt teaches law at the University of Pennsylvania's law school.