Divorce, or not marrying in the first place, might seem like a more logical response to a desire for openness. But even as marriage rates have declined in this country, the institution has retained a seductive status for Americans. In his 2010 book, “The Marriage-Go-Round,” Andrew J. Cherlin, a sociologist at Johns Hopkins University, argues that Americans, who are more religious than their counterparts in other wealthy, developed nations, are also more infatuated with marriage. And yet the tradition is nonetheless at odds, he argues, with the country’s emphasis on individualism, a tension that leads to high rates of divorce but also to remarriage, with worrisome outcomes for finances and children. Openness in a marriage, for better or for worse, would seem a natural outgrowth of those conflicting cultural values, especially since same-sex marriage, open adoptions, single-parent homes, and ideas about gender fluidity have already redefined what constitutes a family. Two-thirds of Americans feel that “a growing variety in the types of family arrangements that people live in” is “a good thing” or “makes no difference,” according to a 2013 survey by the Pew Research Center.

And yet open marriages — and to a lesser degree open but nonmarital committed relationships — are still considered so taboo that many of the people I interviewed over the last year resisted giving their names, for fear of social disapprobation and of jeopardizing their jobs. It is no surprise that most conservatives would perceive the concept as a degradation of marriage, of a key foundation of society. But even among progressives I talked to, the subject typically provoked a curled lip or a slack jaw. The thought bubble, or expressed thought: How? How could any married person be comfortable with, or encouraging of, a spouse’s extramarital sex? The subject seemed offensive to many at some primal level, or at least ridiculously self-indulgent, as if those involved — working, married people, people with children — were indecently preoccupied with sexual adventure instead of channeling their energies toward, say, their children, or composting.

Married for 14 years, I felt that same visceral resistance, an emotion so strong it made me curious to understand how others were wholly free of it, or managed to move past it. The divide between those who practiced open relationships and those who found the idea repugnant seemed inexplicably vast, given that members of those two groups often overlap in the same relatively privileged demographic (anyone holding down three jobs to keep a family together is not likely to spend excess emotional energy negotiating and acting on a nonmonogamy agreement). The more I spoke to people in open relationships, the more I wanted to know how they crossed a line into territory that seemed so thorny to their peers. I interviewed more than 50 members of open marriages, some of them a dozen or more times. I was drawn to the couples who were just starting out: What would the following months bring, what would they learn about themselves? I knew I wanted to follow the arc of their marriages, but I underestimated what, in so doing, I might learn about my own.