Working on my new book, I spoke with dozens of long-married people who described themselves as monogamous. And yet, when pressed, most of them offered something along the lines of, “Well, there was this time . . .”

They didn’t cheat, had zero tolerance for cheating, would kill a cheater dead, and yet . . . they would admit that they get crushes, once kissed someone at a work party, watch a lot of porn, have intense friendships with members of the opposite sex, slept with someone on vacation once but will take it to their grave, had a disastrous affair 10 years ago and it took a lot of therapy to trust each other again, went through a phase where they and their spouse had threesomes. There was nearly always something.

This is the messy truth of many a long marriage: There are temptations. There are negotiations. Sometimes there are even affairs, whether condoned, forgiven or secret. Sometimes they aren’t such a big deal. Sometimes they are devastating. But even then, they don’t always end the marriage. Some people have even found their marriages enlivened by extramarital intrigue. There are as many recipes for figuring this out as there are marriages. It’s messy, but so is life.

As a nation, we hate cheating. A lot. A poll by Gallup found just 9 percent of US respondents considered infidelity morally acceptable. Contrast that to the number that said the same about divorce: 72 percent. But other research has found that somewhere between one in 10 and seven in 10 married people have cheated (people aren’t always super straightforward about this stuff when interviewed by researchers, so it’s hard to get great numbers). Given how disruptive divorce can be, and given our longer lifespans, shouldn’t we rethink our conviction that cheating is worse than divorce?

The way we talk about it these days, if you stay married from age 24 until death at 90, during which time you have a couple of affairs, your marriage is a sham, but if you are faithful to each of several spouses over the same time period, then you’re a moral beacon. Monogamy, says Belgian psychotherapist Esther Perel, “used to be one person for life. Today, monogamy is one person at a time.”

We need a better way to talk about extramarital desire that’s neither “We gotta be free! Our biology compels us to sleep around! Rah-rah polyamory!” nor “Once a cheater, always a cheater.” Monogamy is a spectrum, not an open or closed door.

At one end, there are “zeros” who have never looked at another person, even on a screen. This works really well for a lot of people. Plenty of parents I know successfully channel destabilizing lust into other things, such as bake sales. At the other end are “fives”: Married people who regularly have romantic and sexual affairs with other people.

As you grow with someone over the decades you have to keep recalibrating what monogamy means for your marriage.

I’ve been married for 14 years and couldn’t have such an open marriage, but I’m also not Mike Pence. I have dinner with my male friends. I reserve the right to flirt with the UPS guy, too. And I refuse to feel ashamed that in the course of the past two decades, my husband and I have both had to grapple with feelings for other people. In my research, I encountered a ton of happily married people, who, like myself, live in the monogamy gray zone, as “twos” and “threes.” One woman I met has been married for 14 years; when she and her husband were first together, they would sometimes flirt or even make out with other people. She credited this frisson with keeping their marriage sexually vibrant.

Then he kissed a libertine friend of theirs and she freaked out: “Before we had kids, it would have turned me on that he made out with a friend of ours at a party. But after . . . What works the first couple of years might not work once you have kids.”

As you grow with someone over the decades you have to keep recalibrating what monogamy means for your marriage. We want excitement but also comfort, trust but also mystery. I’ve come to see this as a central, ongoing challenge of marriage, one that can be as enlivening and productive as it is scary.

Nonmonogamy is hard. Monogamy is hard.

Expecting that once the ring goes on, you and your partner will both automatically and effortlessly be “zeros” for life is unrealistic. We should ask each other when we marry, and then again periodically: Am I a “zero” married to a “five”? Am I a “three” married to a “two”? And are we committed to doing whatever it takes to making the math work for the rest of our lives?

Ada Calhoun is the author of the new book “Wedding Toasts I’ll Never Give” (W.W. Norton & Company; out now) and the critically acclaimed New York City history “St. Marks Is Dead: The Many Lives of America’s Hippest Street.”