You’ve just found out that your partner has been unfaithful. What do you do next, assuming you want your relationship to survive? If you are Sienna Miller, you issue a set of strict commandments to the errant Jude Law, including staying faithful, controlling his temper and letting you see your own friends. Problem sorted, though it might not be so easy to stick to your own rigid rules, as even Miller has apparently discovered after her own alleged affair with the actor Daniel Craig, who is one of Jude’s mates. What a mess.

But away from Planet Celeb, how do people cope? Often, the reaction is quite the opposite: to give errant partners more freedom. Philip Helyer, a 39-year-old business owner from the Home Counties, discovered his wife’s affair in July this year by casually glancing at her text messages. His initial reaction was complete shock. “It’s like finding out that somebody in your family has died. You feel it physically: it starts in your stomach and moves up into your head. You go red. You feel close to fainting, the room begins spinning, you start shaking.”

A lot of men might have felt that there was no salvaging the situation: 48 per cent of divorced men blame the marriage breakdown on either their own or their wife’s affair. But Philip was determined that his marriage should fall on the right side of the statistics. These also show that your relationship has a better than even chance of surviving an affair: only a third of affairs result in meltdown for the original relationship.

Philip started from what he describes as a conservative belief in marriage. “An affair is quite a small matter compared to the institution of marriage. The convention is that if an affair has happened, that’s the end of everything; the whole thing has just crumbled in seconds. I found out that is not the case at all.”

He rang his closest family confidante, an aunt who works as a psychotherapist. She gave him a way of seeing the challenge differently, saying that even within marriage each partner is entitled to keep a part of their life private from the other, even to the extent of taking a lover.

Philip came to believe that the affair was a symptom that Libby, 37, whom he describes as “stunningly gorgeous and glamorous”, had been feeling imprisoned by the demands of their two small children and needed more freedom, not less, in her life. The couple spent weeks talking things over and gradually things started to improve. “She started saying that she sees me in a different light now. We talked about how we should deal with things in future, having these private lives and respecting each other’s space. Once your spouse is out the door, you’re not concerned with what they’re doing, as long as they stay safe.”

After safety, the next rule is dignity; or as Libby says: “You don’t flaunt it. We’ve agreed that nothing happens in our home town.” A further part of the new agreement is that Philip doesn’t ask and Libby doesn’t tell him — or anyone else — whether the affair is over. Nevertheless, Philip still feels that trust is an important element, although he has a new definition for it: “You trust them. Not that they won’t have sex with someone else but you trust that they will still come back and they still love you. That’s all that somebody can ask for.”

He is almost evangelical about this arrangements but he is uneasy about referring to it as an open marriage. “Unfortunately, it seems to have quite a kinky ring to it. Also ‘open’ might sound as if it’s more vulnerable. But perhaps this type of marriage, where both people have their own private lives, is more secure. It’s not turning a blind eye, being in denial or sweeping it under the carpet: that is very detrimental. It’s not a loose relationship. I would call it a protected relationship; a marriage with insurance, like an armour-plated marriage. Nothing from outside can destroy it because you have marriage and freedom.”

Philip and Libby are perhaps merely articulating a philosophy of selective tolerance that has sustained many marriages in the past. Such an attitude may, for instance, help to explain other quirks in the statistics, such as the class bias: the lower a woman’s social class, the more damage an affair tends to have (see below left). Upper-class marriages have long borne out the adage: “Fidelity is nothing, what matters is loyalty.”

But, even in Philip and Libby’s case, it seems that the emotional burden weighs more heavily on the female side. “It’s odd,” says Libby. “You can either deal with an affair by letting the marriage fall apart or you can accept that an affair can take place within a healthy marriage, which is what we’ve done. I’m very up and down with it all. Sometimes I feel happy that my whole life and my children’s lives haven’t been turned upside down. Philip appears very happy and I’m going off two or three times a month feeling like a free agent. It sounds perfect.

“Other times I don’t know if my personality lends itself brilliantly to the idea of having a full-on relationship with two men at the same time. Emotionally it’s very hard. I do feel a lot closer to Philip, though. He says very insightful things that make me think that he does understand what I’m going through.”

Philip is convinced that this is the way forward for marriage in general. “I don’t know if it saved my marriage. Certainly I think it has improved it. For one reason or another, no marriage is safe but, certainly, affairs per se are a very poor excuse for a divorce. I think countless marriages would be saved if people had different attitudes about affairs.”

Appreciating those possible differences is the key, according to Relate, the UK’s largest relationship support agency. Denise Knowles, a Relate counsellor, says: “Every couple are different and need to negotiate their own rules about what they will and won’t accept within a relationship.”