Our first date with Geoff and Arlene had been a qualified success. That is to say, Tilly had had a good time with Geoff, allowing him to give her anal, and Arlene and I had eventually come to terms with each other to the extent that I had fucked her and rimmed her to orgasm.

That was in separate rooms at their innocent little suburban home. Then, as Tilly and I discussed it afterwards, we had found ourselves agreeing that it might be fun to have all four of us in the same room. Tilly sent Geoff a Whatsapp; she was going to phone them, but at the last minute we thought messaging might be better, because it gives people time to think.

Great time, the message said. Do it again this Saturday?

They responded quickly. Same room this time?

Exactly what we were thinking, we replied.

Cool, came the reply. Our place again, okay?

Sure, fine, we said. It was Tilly's account to Geoff's, although on this occasion I was co-writing and I felt Arlene was probably in charge at their end, because she was the more communicative of the two.

There was an unspoken agreement that we would keep it couple-to-couple, which made sense in terms of avoiding the complications of one swapped pair getting too involved. But I did feel something for Arlene in a strange way that contained protectiveness because she was so innocent in some ways, so unworldly, and so was he. But there was also something comforting about her; in her own way she had confidence and she thought I was this intellectual who lacked the common touch and could benefit from her being less educated, less traveled. She felt she was closer to her inner self than I was. So although we had engaged in a practice that was outside her experience and she probably wouldn't tell her best friend about, ever, she was taking responsibility for allowing it, even though it had been my idea to put my tongue between her buttocks in a way that was clearly, obviously weird and went against everything British people were taught about decency and hygiene, reducing so-called intelligent human beings to unthinking primitives led by primeval instincts that said a lot about the origin and evolution of the species.

But that's putting my interpretation on it and expressing it in a way that she would consider over-intellectualised.

On the Thursday afternoon, she took a risk. She sent me a message. Is Tilly there? I have a ? for her.

Just me, I'm afraid, I replied. Can I help?

Girls only, she said.

Okay, I replied. Looking forward to Saturday. I added an emoji of a boy with a halo.

That meant to be you? she replied. I don't think so, naughty. She added the one of a girl wearing a mortarboard, the symbol of academic graduation.

Shall I ask her to message you? I asked.

Ssshhh! she said in conclusion, and I decided not to mention it to Tilly.

But as soon as Tilly came in, she said, “Geoff sent me a message.”

“Oh yeah?”

“Yeah. Said Arlene had been asking questions.”

“What kind of questions?”

“About whether I'm more exciting than her. I told him that was dangerous ground and maybe we should reconsider meeting again.” I sat up.

“Oh, no, that would be a shame,” I said, trying to keep it neutral.

“I know,” she said. “I said we'll have to be careful. Maybe she's not so confident under that determinedly unpretentious facade.”

A wave of relief swept over me that Tilly saw it in that light. It would only invite trouble to reveal my little exchange with the lovely Fang.

Tilly is more pragmatic than I am and I was sure she saw the swap in the same light we had originally envisioned: pure, uncomplicated adult fun, not much more than going to the cinema or for a walk. More intense, of course, but with no real consequences.

It occurred to me that since the highlights in both bedrooms had been far from mainstream, but on the kinky side, that was just sex, with little emotion involved, and therefore we were safe.

“Same room,” I mused. How many beds were in your room?”

“One,” Tilly said. “Standard double, quite comfortable. How many in yours?”

“One,” I said, “but it's big. King size.”

“So two... episodes could be going on independently at the same time?” she probed.

“Yes, I was wondering why they didn't suggest a hotel,” I said.

“Exactly,” Tilly replied. “Two double beds. But they're real homebodies. They feel safe there.”

Saturday night came and this time we took some extra booze: tequila and triple sec plus some limes, so I could make margaritas We briefly considered taking some grass, marijuana which a friend had grown in his greenhouse along with the figs and Chinese gooseberries, but on reflection, we thought Geoff and Arlene would be somewhere between horrified and terrified at the presence of what they would classify simply as “drugs” without differentiating between a casual, relaxing smoke and something more dangerous. They had rubbed along in the same world as us for the same length of time but had been on a very different path – and they were looking good on it, so good for them.

“Oh my God, margaritas,” Arlene squealed when I took out the contents of the bag. “Remember that guy in Lanzarote?” She and Geoff tittered and made faces before explaining that getting drunk on tequila was not the same, because it was made from cactuses, which are hallucinogenic. Undaunted, and in the spirit of mutual development, of which swapping was part, I knocked up two of the sharp little cocktails and Tilly and I sipped them approvingly.

“You don't just knock them back?” Geoff asked. “They're shots, aren't they?”

“Mix them with things and they're a cocktail,” I said. “Much more civilised.” I regretted the word as soon as it was out.

“Ah, Einstein's here again,” Arlene chirped. “Good thing I know you a bit better now. By the way, I looked up what you called me, Miss Didactic, and it's not Latin, it's Greek. Ancient Greek.”

“You may well be right,” I conceded.

“I am right, she said triumphantly. “I can use a dictionary and I've got one on my phone that tells you where the name comes from. So there. Not just a pretty bottom.” She looked at Geoff for approval but found that he was gazing at Tilly.

“Very good,” I said. “Brains as well as beauty.”

“Right,” Arlene said. “Hon. Are we going to get started?”

“Aye, good idea,” her husband replied, and led the way to the downstairs bedroom. The curtains were drawn tight, the lights off and just a couple of candles flickering safely in the corners. Arlene closed the door behind us and there we were, all of us in an unfamiliar situation. I gestured to Tilly to go over to Geoff and they began undressing each other. Arlene came over to me and we kissed and caressed each other. When I looked over at the others, they were fully naked and Tilly was on her knees giving Geoff a blowjob. His cock was respectably big but not huge, as I found myself noting with something like relief. I was happy with my own, but, you know...

“Mmmm!” Arlene said. “You want some of that?” She pushed me down onto the bed and flat on my back. “How's Mr Sensitive today? Oh, looking good,” she added, her breasts resting against my thighs as she sucked me. She manoeuvred us over to one side and the bed rolled like a boat as our spouses landed and assumed the same positions. Arlene seemed to enjoy the symmetry, or perhaps the camaraderie, as two women gave head to two men. She reached across and I thought she was going to touch Geoff's leg, but her hand landed on Tilly's arm and she squeezed it gently.

Then Arlene climbed up me and whispered in my ear.

“I want you to do what you did last time”

“Which bit?”

“Licking my bum,” she said, kissing me quickly before pushing me aside and kneeling with her bottom in the air. The movement next to us stopped and I knew her husband and my wife were watching as their partners engaged in what in other circumstances might have been an act of base betrayal.

In the corner of my eye, I saw Tilly climb up Geoff and crouch over his face. It was all I could do to concentrate on my heavenly task as I became aware that Tilly was releasing a short and commendably controlled stream of urine into the astonished man's mouth. Arlene was watching too, and it seemed to add a level of animality to her groan as my efforts took her closer to orgasm.

Then Arlene suddenly snapped, “Okay,” and turned onto her back. As she pulled me into her between her widely parted legs, I saw Geoff and Tilly doing the same. Arlene was wrapped around me like an octopus and grinding smoothly, kissing me with a fabulously warm tenderness. The other two were similarly entwined at crotch and mouth and there was a lot of what felt like emotion in the air. Geoff and I seemed to synchronise our movements – or was it the women dictating the pace?

An arm snaked out across the sheet and Arlene and Tilly were holding hands. Then Tilly's hand was vying with my cock for the rights to Arlene's hot, cooking pussy. Arlene in her turn found her husband's slapping balls and held them as he pummeled my wife's crotch.

The intensity grew more and more until Tilly squeaked in a way I knew very well as she ground herself against Geoff and his face contorted as he came violently inside her.

Arlene wrapped her legs more tightly around me and pulled my head down for an intense, powerful kiss as she came, and I unleashed my stuff into her as if it were a lifetime's accomplishment.

The four of us lay still for half a minute before the girls suggested with subtle movements that we might uncouple. As Geoff and I withdrew, Tilly rolled on top of Arlene and they kissed tenderly.