When Kate Bingham's family went on holiday, her parents' lovers came, too. She soon learnt that true love feeds on hypocrisy, compromise and betrayal
I have often thought this would make a fantastic pre-credit sequence for the next Four Weddings and a Funeral. It's the week before Easter, 1979, and an ancient silver Mercedes sets off from London for Chamonix, in the French Alps. These are the days before roof-racks. Two pairs of men's skis have been slotted inside the car; their tips reach back as far as the rear window. Things aren't too uncomfortable for the two in the front. Luxurious, you might say, compared with the lot of the four in the back. Motorway signs flash by as the camera cuts from face to face (no one is smiling), and the full horror of the situation gradually reveals itself. This year Mum and Dad have brought their "special friends" on holiday.

Details of how, exactly, we survived that three-day journey have been blotted out. All I remember is the row that exploded when we arrived at the hotel to find that only one of the rooms had a double bed. Dad and Julia got it in the end. We didn't see much of them for the rest of the week.

This was the holiday I learnt gin rummy. Mum, my brother John and I played hand after hand, while Sam nodded off in the corner. I don't remember meeting Sam. I was too young, presumably, to register his significance. I think Julia came a little bit later: from the back of the car (a different car, a Mini this time), I recall watching her giving Dad a big kiss on the pavement. She was wearing a Fifties rock'n'roll skirt and a tight white T-shirt. Her hair fell, as they say, to her waist. And I wished Mum would grow hers as long as that. I was six, Mum and Dad were 32 and 33 respectively. Julia was 22. I saw a lot of both "friends" after that.

To use a Nineties phrase, Mum and Dad wanted to have it all. They believed in staying together for the sake of the kids, but they didn't want us growing up surrounded by lies, hypocrisy and bitterness. They had fallen in love with other people, and, having smashed their marriage vows, decided at least to be honest about the fact. Looking back at the set-up, it seems astonishingly naive. They lived in the same house, ate at the same table, even slept in the same bed every now and then and, somehow, expected everything to be all right,

In a way, everything nearly was. Julia and Sam never moved in with us, but having them around was a bit like growing up in an extended family. If Mum didn't have time to take us swimming, there was always a chance we might persuade Sam instead. Julia helped Dad make sure I did my homework. And there was always someone around to do the babysitting, someone's arm to twist for sweets. In spite of the disastrous Chamonix experience, the six of us sometimes went for weekends in the country. While Mum took charge of the main course, Julia made apple crumble, rice pudding, treacle tart.

But even liberals need rules. As the conventional marital boundaries dissolved, a new, unwritten, constitution slipped into place, preserving the delicate status quo. Dad got twitchy if Sam came round more than three nights a week. And Mum would mutter dark ultimatums as to what would happen if Dad took Julia to meet his parents. When it came to parenting there were certain tasks both "friends" took care to avoid. Paying for a round of pizzas at the local take-away counted as helpful, but driving John to A&E because he'd swallowed an olive stone was the sort of thing only a parent could do. Our Christmas and birthday presents were monitored for price and popularity, and woe betide any "friend" daring to creep upstairs at bed-time to tuck us in.

Suddenly John and I had a part to play. Whereas Sam showed little interest in us (he had children and an ex-wife of his own to think about), it seemed as though Julia might muscle in. Sometimes, when it was Dad's turn, she'd pick us up from school instead. And the thought of her cuddling John (a very pretty, dark-eyed four-year-old) made Mum feel sick. Both parents expected a degree of loyalty, and, like the children of so many split families, I became a diplomat. Mum needed John and me to be comfortable with Sam but never came to terms with the thought of us relaxing en famille with Dad and Julia. I knew better than to talk about those times. Answering even the simplest question from Dad ("How's Sam?" for instance) required consideration and tact. Quite unintentionally, my parents transferred the burden of hypocrisy to me.

Things got harder at school, as well. How should a seven-year-old describe her mother's boyfriend? Some of the kids in my class had step-parents, others had mums and dads who were divorced. I wished I could have used those words. Instead I found myself explaining the situation in terms of who loved whom, which nobody seemed to understand at all. If I had been older, perhaps I would have had the sense to lie.

In spite of the problems, I have never regretted the way my parents chose to arrange our lives. It was an awkward set-up, but it lasted eight years. I learnt about sub-texts and betrayal and compromise. And most importantly, I learnt about love. For love was always the great excuse, the principle they had both refused to compromise.

Most children grow up surrounded by talk of love and they soon learn about the different kinds, particularly sexual love which is, of course, hugely embarrassing. In families like mine, where the parents are involved in open love affairs with other people, there isn't a convenient place for that embarrassment to hide. I quickly learned to knock on bedroom doors. The symptoms of romantic love - passion, jealousy, peculiar afternoons of blissed-out happiness - were just facts of everyday life. In the end, of course, Mum and Dad got divorced. Too many promises had been broken and, as John and I grew older, staying together for the kids seemed less and less important. It was a relief to us all.

The evidence suggests children from broken homes are themselves more likely to divorce. As a survivor and beneficiary of that process, I suppose it's true I am less frightened by it than some. Even so, when my husband proposed, I didn't need to think about the answer for more than a second or two. We have been together for nearly eight years. It's tempting fate to say I've learned from Mum and Dad's mistakes, but it's also not strictly accurate. I learnt from Mum and Sam's and Dad and Julia's mistakes as well. For that, I will always be grateful.

There's an interesting, if incidental, detail to this story. The night my parents met, in the student union bar at Birmingham, Dad lent Mum his well-thumbed copy of Anna Karenina. For an exposition of the seductive and destructive power of romantic love, you can't do better than study that great novel.

'Mummy's Legs' by Kate Bingham is published by Virago, pounds 9.99