Epic rage comic by Ilkka Kokkarinen. Click to view full size.

Some of this commitment talk sounds sweet, and some of it, like “committed, interdependent partnerships between consenting adults,” sounds more like a real estate transaction than a marriage. But for Mr. Blankenhorn, these definitions miss the point. He is amused, for instance, at their neo-Victorian avoidance of any mention of sex. Similarly, these definitions dodge any mention of children and parenthood. They emphasize marriage as private and too diverse (“unique”) to be pinned down.

On the contrary, Mr. Blankenhorn writes, marriage is a “social institution,” a set of shared understandings and public meanings that shape expectations and conduct. Marriage has evolved and, yes, may be “constantly evolving”; here Mr. Blankenhorn moves through biology, prehistory, history and anthropology, from ancient Mesopotamia to the Trobriand Islands. But marriage fundamentally involves sexual intercourse and the affiliation — emotionally, practically and legally — between any child created and both parents.

“If this book had a subtitle,” Mr. Blankenhorn writes, “it would be ‘An Argument About Institutions.’ ” He captures his ideas of marriage as an institution with a quotation from a wedding sermon that the German theologian Dietrich Bonhoeffer sent to a young couple from his Nazi prison cell. Bonhoeffer, soon to be executed for his role in a plot against Hitler, wrote, “It is not your love that sustains the marriage, but from now on, the marriage that sustains your love.”