THE SEX DIARIES: Mark dropped to his knees and worshipped every inch of my body. I tried to block out that he was another woman’s husband…

At a recent 60th birthday party, I was introduced as “sexy single mom.” I’ll take it, even though the other guys’ few line introductions included their job titles.

Or maybe that’s my job title now – at least the husbands at that party seemed to think so. As the alcohol flowed, one of them, who I’ve known distantly and platonically for years, put his hand on my butt as we waved at the iPhone speaker and didn’t turn it off all night.

I was drunk enough not to mind. Actually, I thought it was funny because his wife – my friend – didn’t care either.

But when he turned to me and said, “I’m way too sober to ask this, but I thought you might be up for a threesome,” I shook my head in surprise.

For long-married couples, I seem to be a once-in-a-lifetime chance at a drunken party; for me it was another evening of feeling like a dangerous loner outside the marital cage. Why would I want to sleep with a man who is out of shape? I’m having the hottest sex of my life with my younger lover Eliot, even though my friends (and their husbands) know it can’t last long.

After the party, as the married couples returned to the comfort of their marital bed, I went home alone.

Yet, early in my relationship with Eliot, before we were “exclusive,” I slept with a married man for the first and only time in my life. (At least the only time I know of. Maybe some married men take off their wedding rings as soon as they leave the house.)

I knew Mark vaguely through my cousin. I didn’t know his wife well. She was a doctor; Mark worked on different projects and made much less money than she did. He was a known flirt, perhaps as a way to assert his ‘masculinity’.

These types of arrangements can be over-the-top these days, with acronyms like ENM (ethical non-monogamy) and dating app descriptions that say “lovingly partnered but open to others.”

However, Mark’s attitude was not like that. I was pretty sure his wife didn’t know. I met him at a dinner party while visiting my cousin in Wiltshire. His wife was away with the kids and Mark was in off-leash mode.

I shouldn’t have let him flirt with me so wildly, or enjoy his hand on my waist as we danced in the kitchen after we finished dinner. I was drunk, but that was no excuse.

It wasn’t so exciting that it was forbidden either; I hated betraying another woman.

But I was still reeling from my divorce, almost fifty, vulnerable, horny and trying to forget Eliot. I was already in love with him, but I knew we couldn’t be that way forever – he wanted children, I already had three.

Mark lived in the same village as my cousin and he suggested we go back to his house. As he walked down a dark country road, he pulled me to him and kissed me.

I enjoyed the desperation in that kiss, if I’m honest. It was warm to feel so wanted, having my heart torn apart by Eliot – and my ex-husband Simon – back in London. “I want to do everything to you,” Mark said. He was older than me, in his mid-fifties, and had slicked back gray hair. He was also smaller than me, but that didn’t matter. Mark was undeniably handsome and charismatic.

When we got back to him, I had a short but stern reality check. The house was clearly a family home; the hall was filled with black teenage puffas and crocodiles, piled on a shoe rack. There were photos of the family on a skiing holiday in the kitchen.

It haunts me that I ignored this and let Mark drop to his knees and worship every inch of my body. But Mark should take more of the blame. He was the married one, while I was (and still am) the casual canon – and I barely knew his wife.

He undressed me on the couch, next to the family’s sleeping whippet. He was desperately excited – maybe all that flirting usually got him nowhere? He had a father’s body, but I have a mother’s body, so I have no judgment on that.

But there was one part of his body that exceeded expectations – the most important part in the current situation – and I’m not talking about his full head of hair. It was extremely impressive – and took some getting used to. I won’t lie, it felt good.

However, having sex on Mark’s couch was surreal. I kept looking at our reflection in the black windows. I looked sexy on top of him – and it was wild, at least until we were done.

I felt guilty then, and still do.

But Mark didn’t. He texted me the next day asking for a rematch. I deleted the message and blocked him. Hopefully he has changed since then. But I doubt it.

• Annabel Bond is a pseudonym. Have names changed.