My husband Stephen and I were dining on the restaurant terrace of a five-star Caribbean hotel with a couple we’d befriended in the bar a couple of evenings before. All well into our 60s, at first glance we would have looked like a pretty ordinary – if not a little dull – bunch.

But closer inspection would have revealed an undeniable crackle of chemistry between Stephen and our new friend, Maggie. And when I reached down into my bag for my phone, I spotted her leg pressed against his.

I felt a bolt of jealousy but didn’t say a word.

Why? After much discussion, we had agreed to open our marriage to the possibility of dalliances with others and this holiday was our chance to explore that further. I didn’t expect Stephen to leap into it quite so quickly but neither did I feel I could complain.

The prospect of an open marriage might sound shocking to some but we’re far from alone in exploring ‘polygamy’, as it’s called. The UK has over 150 swinging clubs with specifically designed venues, adapted hotels and leisure clubs – with approximately 1.5million Britons regularly taking part.

I cannot pretend it was my idea or that I agreed without reservations but, having both been widowed before we met, Stephen and I had agreed to take more chances; to grab life by the scruff of the neck and properly live it. After all, we knew only too well how short life could be.

So we took this path into the unknown – but not without regrets.

While some brag about the thrill of being so daring, they neglect to mention the more mundane issues it can leave behind. Namely jealousy, paranoia and the risk of the relationship breaking down altogether.

It couldn’t be further from my first experience of marriage. I was married at 21 and had three daughters. My husband, an engineer, was a decade older and far more conservative. We never talked about sex, our fantasies or did anything other than the missionary position.

We were happy, until he died suddenly of a heart attack at work. My daughters were in their 20s with their own lives by that point but we were all absolutely blindsided by his loss.

Then 53, it took me the best part of five years to grieve and even start to think about dating. I’d only ever had sex with my beloved late husband – and hadn’t even imagined being with anyone else.

The dating apps weren’t for me so I joined a widows’ group, where I met Stephen. Five years older than me, he had lost his wife five years earlier too.

We had a similar sense of humour and shared a love of dogs. We started out as friends and took things slowly, dating for three months before we became intimate.

It was wonderful to have such an enthusiastic response to my post-menopausal body. Stephen had experienced a similarly safe sex life during his marriage – missionary position, once or twice a month – and you could count on one hand the number of sexual partners between us.

Together, though, we had a chance to explore our fantasies. My eyes were opened by books such as 50 Shades Of Grey but never thought I’d be able to indulge in such things.

Stephen awakened a sensual side I hadn’t known existed. I discovered I enjoyed buying sexy underwear and Stephen loved checking us into hotels under different names for naughty weekends.

After two years, we married and Stephen moved into my home. At 60, I was still working part-time in admin for a building firm while Stephen had taken early retirement from a nursing career. He had a son from his previous marriage who lived overseas.

It was Stephen who first mentioned opening up our relationship. We were having sex when he whispered that I should picture the type of man I’d like to be intimate with. I was very turned on by the idea and went along with the fantasy. Stephen confessed he liked the idea of me being with someone else.

A couple of weeks later, over a coffee in a cafe. Stephen suggested we give serious thought to swinging. At first I thought he was joking… and was too shocked to reply.

But back home, I went online and found hundreds of message boards filled with sensible advice on having partners outside of marriage. The general consensus seemed to be: set ground rules, go to an established club and ‘look before you buy’.

Far from being a scandalous secret, I was surprised by the number of people our age espousing the virtues of open relationships. It sounded like it spiced things up in the bedroom. As naive as it sounds, it didn’t occur to me that Stephen might have his head turned by another woman – after all, he’d said it was the idea of me with someone else that appealed to him.

A couple of years passed before we actually decided to give it a go. Stephen was having problems maintaining an erection in bed and had become withdrawn and bad tempered.

When he mentioned a local swinging club, I was pleased to see he’d perked up so agreed to go with him. The ground rules were that it was all about me being with someone else, not him – but I didn’t want him watching me. I worried he might get jealous.

The club was an hour’s drive away, hidden in a detached house in a rural area. I wore a clingy black dress and was amazed to discover middle-class couples just like us, well-dressed with decent cars and not a tattoo or piercing in sight.

While Stephen enjoyed the attention of other women at the bar, I had a drink with a tall, well-built man, who was ten years younger than me. Within the hour, we had gone to a side room together.

I was extremely flattered and more than a bit excited. Stephen waved me off with a smile – I know I should have felt a pang of guilt, but I didn’t.

After several minutes of kissing, we had sex. I must admit the reality was far more awkward than the fantasy. On the one hand I felt stunned at what we had done, on the other utterly elated.

Quizzing me about what had happened the moment we left, Stephen was delighted. He was amazingly turned on by the idea of another man coveting his wife.

After the initial high, however, I felt flat. I kept thinking about the guy and wondering if he thought about me. I berated myself for not even asking his name. It’s just as well I didn’t know it because every time Stephen and I had sex, I’d think of that tall, handsome younger stranger instead.

The second time we went, a few months later, I was intimate with another stranger. I’d gone in the secret hope we’d bump into the same chap as the first time but we didn’t. This time, the man in question asked if Stephen might want to watch, but I told him absolutely not. I shuddered at the thought – somehow that felt like more of a betrayal than having sex with Stephen not there.

Once again, Stephen was tremendously turned on when we got home. He hadn’t done anything at the club, and I’m not sure how I would have felt if he had. Besides I was doing this for him – with hindsight, however, we should’ve restricted our fantasies to our imaginations.

We went a couple of times more but I didn’t sleep with anyone else. There were plenty of admiring glances, which Stephen loved, but opening our relationship had served its purpose. Stephen’s libido returned and our loving, sexually charged relationship was back on track.

A year on, however, Stephen hit me with a bombshell. He wanted to give it a go too. Not at the same club but on holiday. As I had slept with a stranger, I felt I couldn’t object to him doing the same.

We booked a fortnight in the Caribbean. The last thing I imagined was for Stephen to start looking for another woman right away – yet he did.

That’s how we ended up having dinner with Maggie and Michel on our third night. Maggie was blonde and very beautiful – but I didn’t foresee the turn things would take.

Her husband, Michel, was French, his accent hard to follow and an utter bore. Meanwhile Maggie had unbuttoned the top of her dress and, a flash of lingerie visible, Stephen was transfixed. Jealousy flooded through me as I realised she wanted to have sex with him.

We hadn’t talked through what would happen if Stephen wanted to go ahead and I did not. We excused ourselves to talk and it was then he told me she’d already invited him back to her room.

This was my sliding doors moment because Stephen asked if I was willing to let him have sex with her. I said yes. But really, what choice did I have?

So off they went.

When Michel proposed we go to my room to see the view, I responded with a very firm no. He was a bit annoyed, but I couldn’t have cared less.

When Stephen returned to our room, he couldn’t stop smiling. He tried to cuddle up to me in bed but I feigned a headache. I didn’t want to know what had gone on and I certainly didn’t want to sleep with him after he’d been with her.

I didn’t say anything when Stephen trotted off at various times of the day (and night) to have sex with Maggie throughout the rest of the holiday, but I felt extremely vulnerable and wanted to go home.

I put up and shut up because Stephen hadn’t stopped me when I’d slept with other men – quite the opposite. But I felt very hurt and worried about the state of my marriage. I tried to tell myself it would be confined to a holiday fling but on the flight home, Stephen veered from pensive to sullen.

By the time we got home, I’d rehearsed my speech: it had been a bit of fun but it was time to close our marriage again. But I was in for a shock. Stephen announced that he had developed feelings for Maggie and it was mutual.

Because life is ’too short’, he was going to stay with her to see how things pan out.

I was utterly blindsided and burst into tears. I couldn’t quite believe that he had effectively dumped me for another woman.

We’d only been back three days when he packed a bag and disappeared off to France. I didn’t want him to go but I didn’t want to beg either. I didn’t tell a soul, pretending to my daughters that Stephen had gone to visit his son.

At home alone, it was torture. I was utterly disgusted that my husband could leave me for a holiday fling. I refused to respond to his calls or messages while he was with her. At the same time, I hoped and prayed he’d come back to me.

Three weeks later, he turned up on our doorstep. They had discovered they had nothing in common and she had no intention of being faithful or leaving Michel. It served Stephen right!

The only way forward for us was by talking. Stephen admitted that while he’d initially been happy about me being with other men, it had turned into jealousy, which in turn developed into feelings of inadequacy. Being with another woman was a way of restoring his ego – or possibly even an act of revenge.

But I didn’t feel sympathy. I asked him how he thought I felt being a third wheel on our expensive holiday of a lifetime. We really gave it to one another. I was very much minded not to take him back.

In the end, though, I did – as I still loved him. It has certainly been a salutary lesson for me; I was out of my depth and will never open our relationship again.

A year on, I know Stephen and Maggie are still in touch via text (which I hate). But I take solace from the fact that Stephen is doubtless one of many she remains in contact with to boost her ego.

It might sound foolish but pride stops me from asking Stephen to cut contact with her. He has reframed their relationship as one of friendship, which I find baffling. I must admit at times I find myself wondering if I know Stephen as much as I thought I did.

Meanwhile, our sex life is undimmed. We still have sex three times a week, but there is no more talk of strangers – and if Stephen needs a bit of extra help we use Viagra.

I have learned the hard way that while you can control a fantasy, you have no control over another human and their emotions.

An open marriage might seem like a grandiose, sexually liberating idea, but it can leave your self-confidence in tatters and your relationship on very shaky ground. That’s if you’re lucky enough to still be together at all.

Juliette Jeffries is a pseudonym. Names and identifying details have been changed.

As told to Samantha Brick.