An open marriage is always destined to end in recrimination and tears, I should know, says BEL MOONEY

Try to imagine the scene. Your two young sons are asleep, you've had another day of being the perfect mum. You love your ­husband very much, and he loves you.

Your sex life with him is excellent, even after ten years. In fact, it's difficult to think of a better marriage . . . and yet. And yet, you want more. You long for thrilling sex with that guy you met in the bar, the one with the slim hips . . .

Tormented by desire and an itch to break free from domesticity, you ­confess your longing for sex with another man to your husband.

His easy-going response rocks your world. He says: 'You can go out with him again — as long as you tell me everything.' You realise he wants to get off on details of your sex with other men. Oh, and he reveals he wants to sleep with other women, too. Tit for tat, you might say.

In More: A Memoir of an Open Marriage, Molly Roden Winter reveals all about her relationship with her husband and the others they share it with

Does this shock those of you who are in committed relationships? Could you do it? Would you ever want to open the door of your marriage, to let others in?

During the early 1970s the concept of the 'open marriage' was current — and some of us even succumbed to the theory that it's possible both to stay and to stray.

Marriage should not be a prison, we cried. We heard the rallying cry of the Black Panther Bobby Seale to 'seize the time' and we viewed personal sexual freedom as a human right. Down with your repressive, bourgeois morality!

People like me (on the Left yet living privileged lives) ­hero-­worshipped Jean-Paul ­Sartre and Simone de Beauvoir — united intellectually, yet staying unmarried and free to engage openly in any number of relationships.

Not that there is any such pretentious theorising in More.

The author and her husband, Stewart, are the next generation — our children, Generation X.

No political postures for them, just sexual ones. With the bare minimum of discussion they agree to their open marriage with mind-boggling ease. Molly has fallen in love/sex with Matt, who in turn feels guilty for betraying his girlfriend. But Molly needn't feel guilty because she's only doing what Stewart agreed to. Meanwhile, he is out with another woman . . .

If it sounds like a complicated mess, that's because it is. I've been there — the moment when you say goodbye to the husband you love and drive off to the ­lover's bedsit. All very 1970. And Molly's account reads like a ­Cosmopolitan magazine confessional of that time. Yet this book is a saga of 21st-century New York. The consensual infidelity Molly and Stewart have chosen is, she explains, 'a double life'.

'In one life I wake with the kids at dawn, make breakfast and pack lunches, manage pick-ups and playdates, cook dinner, run baths, read bedtime stories and sing lullabies.

In her book, Molly Roden Winter identifies 'the void' in many women's lives as 'a need for ­something marriage and motherhood cannot fill'

'I greet Stewart when he comes home late in the evening, give back rubs or make love, and sleep by his side. I am the picture of the dutiful mother and wife — but only in service to my other life, the one where I think about Matt. I picture what he's doing and who he's with. I wonder if he's thinking of me, too.'

What's going on? Like many women (even if they don't admit it) Molly Roden Winter identifies 'the void in my life' as 'a need for ­something marriage and motherhood cannot fill'.

She wants to be 'seen with fresh eyes' and recognises her yearning as 'feeling not like a wife or a mom but a desirable woman again'.

In the movie of this book (bound to be made) that's the cue for Try A Little Tenderness on the soundtrack.

But no matter how many women have those feelings from time to time, few would want the complicated life this book describes.

Molly has gone back to teaching and is a good wife, mother and daughter to elderly parents, who (she discovers) had something of an open marriage themselves.

This is almost the most fascinating part of the book. A therapist suggests there are two sides to her: the 'Straight-A Molly' who always did what she was supposed to do, and the other Molly, the 'badass' woman who wants to be seen for herself, wants to transgress.

But neither of those 'two' Mollys can bear it when Stewart has sex with another woman (his ex, to make it more complicated) and hates the cool way he reminds her that she gave him 'permission'.

They set out some ground rules for their respective liaisons, which include not falling in love with any of the other people. Very wise. But how can you be sure that you won't?

Matt lasts for four years of shame (her), guilt (him) and good sex. There's some more angst from the author, but she decides she still wants to 'expand my sexual horizons'.

A dating website offers Mike, who's a failure; Leo, who is kinky; weirdo Laurent; then Liam. Meanwhile, Stewart is seeing four women at once, and Molly takes advice from a book helpfully titled The Ethical Slut.

When the dating site offers up Karl, Molly finds herself in trouble. She likes him too much. Is this love?

It turns out Karl's partner, Martina, is a female version of Stewart and wants to know all the details of his sex with Molly.

So the affair rolls on until Molly is persuaded into a rather awkward threesome with the pair of them. And then she's dropped. Next is a hook-up with Scott. But enough!

More is a riveting read, but it leaves you wondering what this married couple have set up for each other in the future. They're still crazy about each other — and the book is dedicated to ­Stewart. Yet there is a whole other volume asking to be written, about their two children, for example, and what damage may be done to them.

An open marriage is all very well when you're young and irresponsible but — take it from me — increasing age usually brings recrimination and tears.