The Supreme Court will hear oral arguments next week on the question of whether the Constitution allows states to ban same-sex marriages. Whatever it decides, there seems little doubt that the U.S. is moving rapidly toward allowing such marriages, and with remarkably little public controversy. Contrast this with the issue of abortion, which has split the nation for 40 years (and counting).

Why the difference? There are three standard answers.

One answer is that the Supreme Court ruined everything.

A forgotten bit of history is that in the seven years before the court decided Roe v. Wade (1973), a significant number of states liberalized their abortion laws. Four, including New York, repealed them altogether. Some people believe that by suddenly declaring abortion to be a constitutional right, the court galvanized the pro-life movement and polarized the nation. According to this view, the court’s premature involvement truncated a healthy democratic debate, and it was the arrogance of an activist court that produced the divisions of the past 40 years.

The second explanation is that the moral issues are fundamentally different.

To many people, abortion is a grave moral wrong, because it involves the intentional killing of a living creature legitimately described as a person (perhaps from the point of conception, perhaps at some later stage). The most obvious justification for restricting people’s freedom is to prevent harm to others. When states protect life, they prevent that harm.

It’s a lot harder to say the same about bans on same-sex marriages. According to this view, the controversy over same-sex marriage is quieter because the moral issues are easier.

A third explanation points to the dynamics of social movements.

Everyone knows that, in recent years, an energetic social movement has been working to legitimize same-sex marriage. This movement focuses on repetition of a simple question, illustrated with human faces: Why should some people — your brothers, sisters, children and friends — be denied access to the defining institution of marriage?

Although many people strongly oppose same-sex marriage, and are willing to give time and money to ban it, they haven’t really created a social movement.

Things are different in the context of abortion: Both pro- life and pro-choice movements have been energized and amply funded for decades — and their opposition both reflects and helps to perpetuate national polarization.

The implication is clear: Same-sex marriages are unlikely to produce anything like the social divisions associated with abortion.

Cass R. Sunstein, a Bloomberg View columnist, is director of the Harvard Law School’s program on behavioral economics and public policy.