“The first 20 years are the hardest,” an older woman once told me. At the time, I thought she was joking. She was not. One of the most challenging aspects of being married, and one of the leading causes of divorce, is something that no one mentions in a wedding toast — how tempted, even in a happy marriage, we are likely to be by another person, romantically or sexually.

I would never be so tacky as to mention at a wedding that assorted studies say something like at least half of spouses have cheated. And yet I wonder if we do a disservice to newlyweds by letting them enter marriage with the expectation that once the ring is on, perfect fidelity will come to them as easily as blenders and a china dinner set.

After 16 years with my husband, I’ve begun to think that being good at monogamy isn’t a characteristic, like green eyes, it’s a skill, like playing a sport. Some people are born with an aptitude; others cultivate one — or give themselves permission not to. Some people never look at another person. Others sleep with everything that moves. But it took me a while to learn this.

A few years into my marriage, an emotional affair almost split us up. I was working full-time at a job I’d grown to hate. My husband, Neal, a comedian, was touring but making little money. Our toddler was opening and closing drawers. He was so cute, and we loved him so much, and yet so much of his care involved repetition. (Fortunately, with Dora the Explorer on, no one can hear you scream.) Neal thought I should be making more time for our relationship, which I found infuriating because I thought he should be thinking about how he could make more money, not about what film we should go and see.

So it should not have come as a surprise, though it did, when one day, while our son was in his weekly split-second at nursery and I was getting dressed to go to a meeting, Neal told me he had feelings for another woman. When he said her name, I flinched. I knew her. She was pretty and familiar. I’d never been jealous of her, and now I felt like a fool. I thought I’d known his mind and heart through and through. Had I known him at all?

“You what?” I screamed. I cried so hard I could barely breathe. On my way to the meeting, I tried to fix my make-up, but my face was so puffy it was useless. When I got to the office, I said it was hay fever. Halfway through the meeting, I excused myself to visit the toilet, where I cried some more.

Soon after this revelation, I threw him out, telling him to sort out his feelings somewhere else. He went to stay with his best friend. Curiously, he also went to talk to my father. “You could leave,” my father told him, “but you wouldn’t fix anything. Wherever you go, there you are. You would just have different problems. Are the problems you have now so bad that any other problems would be better?”

“I still love you,” Neal told me. “I love our life together. I don’t want to leave you. I just don’t know what to do with these feelings for her.”

“Kill them with fire,” I suggested.

Neal stopped seeing the other woman. We talked about it ad nauseam. He suffered. I suffered. She probably suffered, too, and eventually I felt pity for her. It seemed that it hadn’t gone very far physically, but the feelings were still intense. Some friends told me I should leave Neal, saying I deserved someone who would never look at another woman. Yet if such a man existed, I doubted I would want him.

I also had to admit that temptation was the least surprising thing that could have happened that year — a year when we were both working a lot, we had a little kid and were apart so often. (I was let go from my job, the one paying most of our bills, soon after Neal revealed his affair. I think we skipped the Christmas card that year.)

We’ve done reasonably OK with monogamy since then, but last year it came up again. When my last book came out, I went on a book tour, doing events at bookshops, churches, festivals, rock clubs, journalism schools and a bowling alley. At one reading, there were only a few people. At another, there were hundreds. People laughed at my jokes, asked me to sign their books and took me out for drinks afterwards. And I loved every second of it.

Before I left, Neal told me I had accomplished something and should see the trip as a victory lap. He said he hoped I would be able to fend off any advances men made on me, and that he’d be mad if I developed feelings for someone else. But he also said that mainly I should have fun.

And so I did.

In one town, I saw an ex-boyfriend for a long talk at a cafe. He assuaged my enduring guilt over how I’d been a lousy girlfriend 17 years earlier. We remembered what good friends we had been, and should be again.

In another town, I went out drinking with some old friends. When I was hugging one of them goodnight, there was a sudden electricity. It took a little while for either of us to let go. “I should get back to my hotel,” I said, finally, and did. The next morning, I woke up hungover and happy.

In Miami, a scandalous friend of mine suggested that we have a threesome with a surprisingly muscular journalist we met at a party. I demurred. We settled for eating fast food on a bench by the hotel.

My childhood best friend, Asia, now a shrink, says you need to figure out how to build sway into a marriage, the way you do into the foundation of a building. She says that just as a tall building or bridge needs to move in stiff winds, a marriage that’s too rigid will crumble at the first tremor.

I had started to feel like the poster child for sway. I could have all the freedom in the world, and look how well I managed it. But my friends in AA quote a line about why you shouldn’t go to bars if you want to stay sober: “If you hang out in barbershops, eventually you’ll get a haircut.”

Butterflies are too delicate an image. The feeling of desire was like bats flying out from under a bridge

One night at dinner after a reading, I showed a friend a photo of a colleague I planned to see on another stop of the tour. Wry and roguish, he resembled something a 3D printer would produce after scanning headshots of my favourite actors.

“Wow,” she said, looking at my phone. “Trouble.”

I met up with this man in his town. Driving towards him, I felt a fluttering of anticipation. Butterflies are too delicate an image. The feeling of inconvenient, insistent desire was more like bats flying out from under a bridge — a black mass of beating wings.

We sat at a bar and the whole time I kept glancing at the place where his arms met his T-shirt. The impulse to touch him was barely controllable. I knew I should shake off the feeling, but I indulged it by staring, like a child queuing at an ice-cream van.

As I went to leave, our hands touched on the table. Soon after, in the amber car-park light of a city where I knew no one else, we hugged goodbye.

“You’re pretty,” he said into my hair.

The bats flapped violently. I wanted to pull this man into the backseat of my car. Instead, I said goodnight and drove back to my hotel alone, vibrating with energy. Once back in my room, full of adrenaline, I turned on the TV and stared at it for a full hour without registering anything. I felt guilty, elated, and I hoped I would see him again.

The next night, I did, this time with other people in a public place. But then, suddenly — and how naturally this seems to happen when you want it to — we were alone again, standing by our cars.

“Will you come over?” he asked. “If you do, I promise I won’t try to make out with you too much.”

Wow, that’s a good line, I thought. He probably uses it all the time. But it still felt tailor-made for me, for that night.

“That’s not such a good idea,” said my brain. “You should just go back to the hotel and go to bed early.”

My body vehemently disagreed. The body is a rhetorical genius and a logistical magician — able to conjure dark corners and empty rooms, able to make insane plans seem reasonable.

“Just stop thinking,” my body said. “Tomorrow, you can come back. You can have all the other days for ever. But for now, shhhhh . . .”

And because in that moment I failed to talk myself out of it, I went. And at his apartment, we made out, but not too much — unless you think that anything when you’re married is too much, in which case it was definitely way too much.

When I got home, I confessed to Neal. He forgave me, and said he also had a confession: the night I’d been doing what I did, someone we know had told him he was attractive and he’d said that she was, too. Nothing more had happened. But I heard the subtext: they wanted each other. He has that bats-flying-out-from-under-the-bridge feeling too, I realised, horrified.

When Neal told me about his exchange with this woman, who exudes a cheerful sexuality that I find insufferable, I was furious. Then I was upset with myself. By doing what I’d done, I’d abandoned the right to be offended. “No affair could ever be as hot as having the moral high ground,” my friend Jason once told me, and while I’d found it funny at the time, now I saw that Jason was entirely correct.

Neal and I wound up spending weeks, maybe even months, processing that one night, unpacking why my kissing and his flirting had happened, and what it meant. He felt a little threatened by what I’d done, but he thought that his own flirtation was no big deal. To him, the mild intrigue with that woman was merely a validation of his virility, but to me it felt monstrous. I hated the idea that this trollop was walking around knowing that my husband found her attractive, that he’d given her that right. This, even though if you’d asked me whether I knew he found other women attractive, I would have said, “Of course.”

“The hardest lesson in a marriage,” says my friend Asia, “is believing in your heart that the other person is as real as you are, and their feelings matter as much. We all think that when something is wrong, it will feel wrong to us, but that’s the biggest lie. So many things that your partner will see as betrayal, will feel to you like nothing. One of the biggest challenges of marriage is to acknowledge that your own feelings aren’t the end of the story. We have to hold so many realities at once: here’s me, here’s you, here’s us, here’s the rest of the world.”

How terribly hard it is to accept that other people feel what we feel. When politicians cheat, we say they are morally bankrupt. When male celebrities lust after their nannies, we shame them on tabloid covers. But when one day at work our eyes meet someone else’s, it feels different. Why do we treat out-of-bounds desire as beyond our control when it happens to us, but as an easily avoided abomination when it happens to others? And why do these things feel so different to us in the moment than they do to our spouses later?

While interviewing couples for my new book — a set of essays that explore the wonderful and terrible aspects of marriage — I kept hearing these stories and these feelings. None of them would describe themselves as being in an “open marriage”, and yet there were occasions when affairs had been either explicitly condoned or forgiven after the fact. Some had done major damage, others had not. Yet one thing had come out of almost every situation: the couple started to talk more about what, in terms of other people, felt safe and what felt sexy and what felt terrifying and what felt like no big deal.

Many couples suggested to me that there is a huge gulf between what they thought they would be experiencing in marriage — perfect, effortless 100% monogamy — and what they had learnt to tolerate or forgive — a kiss here or there, rampant viewing of porn, close friendships with members of the opposite sex … something.

If marriages exist on a monogamy spectrum between totally closed and totally open, with 0 being never leaving each other’s side or looking at another person, even on a screen, and 5 being extended romantic and sexual affairs with many other people, a lot of people are 0s just like they promised they would be when they got married. And some are polyamorist 5s. But many of us find ourselves, at one point or another, by design or accident, navigating the 1-4 range. It’s a truth often ignored that, while sometimes a flirtation or crush can blow everything up, other times it can stoke the sexual fire of a marriage. I think that’s partly because it introduces an enlivening element of danger.

“Paradoxically,” says the renowned psychotherapist Esther Perel, “the things that nurture love are sometimes the very same things that flatten desire. Family life wants consistency, repetition and routine, while the erotic thrives on novelty, the unpredictable and the unexpected.”

Marriage might just involve finding a balance between boredom and jealousy, safety and danger

I’ve noticed that Neal and I need distance to feel attraction. If we’re too connected, there’s no space to bridge with desire. If we’re too far apart, we become estranged. I’ve begun to suspect that, despite what women’s magazines tell us, there might be no perfect way to reach peak sexiness and perfect security simultaneously, that marriage might just involve finding and re-finding a balance between boredom and jealousy, safety and danger.

One woman I met in California, who has been married for 14 years, started out her marriage in the 3 zone. When she and her husband were first together, they would sometimes flirt or kiss other people. As their marriage changed, that became less acceptable for her, something she realised when he kissed a libertine friend of theirs. “Before we had kids, it would have turned me on that he made out with a friend of ours at a party,” she said. “But what works in the first couple of years might not work once you have children.”

Ultimately, she wound up feeling grateful for the disruption. “I realised I wasn’t giving my husband what he needed. I was paying a lot more attention to our new baby than I was to him. I realised that I want to be the person to make my husband feel good.”

After Neal’s affair, I felt something like that too, once I’d got over the shock. It made me sad that he’d been lonely, and it made me face the reality that I’d been lonely, too.

These days, I don’t enjoy crushes as much as I used to. That’s not how I want to be with other people. In her short story Floating Bridge, Alice Munro writes about times “when anything you look at is just a peg to hang the unruly sensations of your body on, and the bits and pieces of your mind”. These other men — did I ever really see them? Or were they just projection screens onto which I could shadow-puppet my own desires?

Besides, trying to have fun but not too much fun can be exhausting, like playing the game Operation, where you can extract the prize only if you don’t hit any of the sensors on all sides. Maybe it’s more like Russian roulette, because for every five times an extramarital flirtation makes you feel extra alive, there’s one crush that kills you.

And the truth is, other people, attractive as they are, will never give us everything we want in life. The modern relationship sage Tyler Perry says that when we’re married, we need to keep in mind the 80-20 rule. Our partner, he says, will give us 80% of what we want. When we look at other people, we see only that they have the other 20%. Of course, if we ever left our partner for that other person, we would again get only 80% of what we want, just a different 80% — new joys, new problems. And looking around, we would still see the missing 20% in other people.

I wouldn’t want to trade in my current problems for new ones with someone else. And yet, sometimes other people are so great. Wouldn’t it be wonderful if we could have our 80% at home and the occasional 20% scattered about elsewhere? And by “we”, I mean me and not Neal.

So this is why I don’t give toasts at weddings: because I would want to say that the happy couple might like to consider adding to their marriage vows: “How much do I love you? I love you so much that I will turn down spending time with someone handsome and fun who I know wants me. I love you so much that if you struggle with feelings for someone else I will try not to hate you for it.” And what bride or groom, amid all the optimistic smiles and white tulle, wants to hear that maybe they should have some strategy for avoiding or forgiving extramarital kisses in car parks?