I have a dilemma. A friend is having an affair. Her (wonderful) husband is blissfully unaware.

I asked her once, my friend with the roving eye, in the way we ask girlfriends these questions after too much wine, “If your husband were having an affair, would you want to know?” Yes, she said. An emphatic yes. She would want to know.

She isn’t alone: I ask other girlfriends, “Would you want to know?” Almost without exception, they say yes. I wonder why. To mete out some kind of punishment? To save themselves from the humiliation of everybody talking about them behind their backs? Because, anyway, why should he get away with it?

You see, I would once have agreed. Yes, I’d have said, confidently, “I’d want to know, absolutely.” Except, when it happened to me – when I was told, “Your husband’s had a thing” – I found I didn’t want to know. Not at all.

I remember so clearly the delivery of that gut-punching news. A friend told me on a walk. I felt winded. I stopped dead. I couldn’t catch my breath, I couldn’t speak. A thing? She couldn’t stop telling me all the details: “It started at a party, somebody noticed they’d slipped away, I’m not sure where you were… ” as if my absence had been the catalyst, as if I ought to have been standing guard, as if it were my fault.

I confronted my husband, of course I did – tearfully. He denied having been unfaithful. He told me I was silly – “You’re being ridiculous” – and stalked off. But if I hadn’t seen the fire, I could taste the smoke; its sourness lingered for ages, tainting everything. His dismissive rebuttal smacked much more of an indignant “How could I have been found out?” than an outraged “How could anybody say such a thing?”

He never confronted the woman who accused him, and I always wondered why not: I would have done exactly that – and immediately. “How dare you make such suggestions?” I’d have demanded. He kept firmly quiet. His silence was deafening and incriminating all at the same time.

Once you’ve been evicted from your comfortable, married-with-children shell of complacency, it’s difficult to get comfortable again. Once a seed of doubt has been sown, it quickly becomes a jungle of qualms, fed by every cold shoulder, every turn of the head. I began to dissect and minutely analyse everything he did, everything he’d ever done. I excavated events from years ago: I thought I remembered how he’d flirted on various occasions, abandoned me for more interesting company. I remembered overhearing him tell an attractive woman whom he met at a party that he wasn’t married. “Pffft, me, married?” he snorted and laughed at the very notion. I was standing behind him, seven months pregnant.

For years after the accusation, I viewed every woman with a brittle, green-eyed gaze, “Why are you looking at her?” I’d demand as he looked into the middle distance, probably perfectly innocently. I was bitter, I made caustic comments about other women – such an unattractive trait in a woman. I stopped being spontaneous, I was a lot less fun. It unspooled my confidence. I unravelled from robust to needy: what was he missing in me that had drawn him to her?

Is it worth abandoning something of substance for something that is a frivolous, transient massaging of ego?

Hearing that he had been unfaithful once infected all our preceding years together and left me sore, raw and smarting until a long time later.

So no. It turned out I didn’t want to know, didn’t have to know. Knowing didn’t add enough to make up for all the things it took away. In fact, knowing added precisely nothing.

Infidelity isn’t rampant. But nor is it uncommon: a gazillion surveys suggest it happens in a third of committed relationships. But it takes so many shapes now – and many of those shapes are of the flaky, you’d-be-better-off-ignoring sort.

And is it worth throwing five, 10, 15 years away when infidelity may amount to a momentary lapse in concentration? Delusion? Distraction? All balls, no brains? Is it worth abandoning something of substance for something that may mean nothing, that is a frivolous, transient massaging of ego?

Later, much later, when I was able to rationalise all of this, when I realised that a brief lapse did not amount to him falling out of love with me, did not mean there was anything wrong with me, I was able to compute it all: to consider the numbers. A night, or two, of foolishness versus the significance of shared years, the partnership of parenthood, the joys, the grief through which we had supported each other, good times and sad that were privately ours. The ballast.

But back to my friend with the unfaithful wife. “Ought I to tell him?” I ask the same girlfriends who insist they’d want to know in the same position. The response to a specific instance is very different. Oooh, they don’t know. Best not get involved, says one. Are you absolutely certain, asks another. How well do you know them both, says a third. The wisest one acknowledges it’s a tricky question: “You’re damned if you do, damned if you don’t.”

So, my question still unanswered, I put my quandary to more friends. Men and women bat the dilemma back and forth. They all agree that it’s a tricky predicament. But nobody knows for sure. Except my husband. He shakes his head: “Don’t,” he says quietly. “Don’t tell. Telling can wreck good marriages.”

It turns out that what they say is true: a little knowledge can be a dangerous thing. My husband’s insight confirms what I think I know is the right answer from my own miserable experience.

The friend who told me about my husband is still a friend. But she is not nearly as good a friend as she was once. And I don’t want to damage a precious relationship of many years with my cuckolded friend in the same way she spoiled ours; I’ll keep my mouth shut.

The woman with whom my husband was meant to have had a dalliance pinched someone else’s husband in the end; he was easier quarry than mine. They have a baby daughter. She nags him a little too often, isn’t as pretty as she was once, seems a bit miserable.

As revenge goes, that’s not bad.

• Alice James is a pseudonym.