The businesswoman and TV personality Saira Khan has revealed she has granted a "sex pass" to her husband of 11 years. Put simply - and I'm sure this is simplification of many considered factors that she will keep private - she has had two children with her husband, Steven, they have a successful marriage, she loves him deeply but she has lost the desire for sex.

The 46-year-old found herself making "excuses from about 6pm", and though their relationship is still intimate (they "cuddle up and it's lovely"), she is happy for him to have sex outside of their marriage.

Her husband denied later that he has any such arrangement with his wife, but the reactions on social media have expressed solidarity and understanding, perhaps proving that more and more people are now open to the idea of reconstructing the traditional model of marriage to suit modern life and human needs.

Many have applauded Saira's decision not to conform to an ancient set of relationship rules that work for some and not for others. Some say they admire her for control of the situation and for opening up an honest dialogue about sex in long-term relationships. The over-used old adage is true; having sex outside a relationship isn't the thing that hurts the most; it's the lying that wrecks the bond. It would be all too easy to pretend the problem isn't there - to pretend her sexual impulses are exactly as they were when they met, out of fear of disappointing her husband. This would lead to frustration on his side, trying to figure out what he's doing wrong and perhaps, in the end, a betrayal.

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There are so many things that can affect a person's libido: having children, stress, sleep deprivation, mental or physical health, body-image issues and, for many women, the menopause. The Journal of Marriage and Family found that 16 per cent of long-term couples, many aged 35 to 44, have "little or no" sex. A survey in February found that one in four couples over 50 never make love. However, the "sexless seniors" agreed that despite the apparently chilly nights, they were still happy with the companionship, conversation and humour in their relationship.

The idea, then, of a "sex pass" if a couple's libido is out of sync, may be the logical conclusion to what is becoming a more common state of affairs for many people. Those who are willing to widen their parameters of fidelity to suit their needs may be showing great bravery and love for each other - as long as both are happy with the arrangement and always honest about it.

This is certainly something Dianne and Paul, both 56, from Yorkshire, can relate to. They have been married for 27 years, and Dianne says: "We used to make love all night in the early days, but we haven't had sex since 2008. We still share a bed every night, kiss, cuddle and have an otherwise affectionate relationship, but I went off sex when I was approaching the menopause."

Like many women, sex became uncomfortable and, eventually, undesirable, but Dianne assumed her libido would come back. When it didn't, she says she had to change the way she thought about their relationship. "At first Paul didn't seem to mind - his libido also waned when he went through a series of stressful work projects. But sex feels so irrelevant to me now that I don't even think it would be a deal-breaker if Paul had it with someone else. I have told him that it would make no sense to call time on my marriage simply because he had chosen to find, elsewhere, something I'm not interested in."

The flux of libido throughout a long-term relationship is not a gendered issue; men have affairs and so do women. Psychologist Esther Perel, author of Mating in Captivity: Sex, lies and domestic bliss, has spoken about how marriage has only recently shifted from an economic structure into a romantic one. She says: "Monogamy had nothing to do with love. Men relied on women's fidelity in order to know whose children were theirs and who got the cows when they died." This shift means we now look to our partner to be so many things: soulmate, parent, confidant, sexual partner, family, protector, provider, intellectual equal...

With this pressure on the marital bond, it's bound to sometimes break. So if you expect infidelity might happen, why not create your own rules so one person doesn't have to provide everything?

New models for relationships are being created all the time; there is polyamory, the act of having more than one loving relationship at a time; open marriages, where both couples have other partners. There are the couples who meet each other's other partners; and those who are happy for one or both of them to have lovers, but choose not to hear any details.

As all of these different models move further into the mainstream gaze, they are coming under more scrutiny. Some think, of course, that the bravery of marriage is in the sacrifices we make, that opening up a marriage is failing and disrespecting the institution. But those who think this are probably less likely to discuss the fact that 42 per cent of marriages now end in divorce and almost 60 per cent of men and 45 per cent of women cheat.

Monogamy isn't working for many - but that doesn't mean they want their relationship to end. Could it be that Saira's decision to give her husband a "sex pass" is simply a confident acknowledgement that they are more than their physical relationship; that there are so many more parts of her that her partner fell in love with: her mind, her spirit, her kindness? As Dianne says: "If we weren't kind and tender to each other in other ways I would worry more. But we are loving and intimate, so I don't think of our marriage as being in a bad place."

Loyalty has many forms, of which vowing to sleep with just that person is merely one. There's the promise to grow old together; the promise to raise children in a like-minded way. And there's the promise to always tell the truth to the person you share everything with. Loyalty can mean physical fidelity; but it can also mean committing to a life of no secrets.

Esther Perel says that when she speaks to a couple who have been affected by an affair, the thing she always says to them is: "Your first marriage is over, would you like to create a second one together?" The same could be said to those who open up their relationship in order for it to survive. It may not be the marriage they entered, but it could still be the one that they end up with.

Some names and details have been changed.