This happened some years ago, when my wife and I lived in the north of England, but every detail stays vivid in my mind. We'd hosted a party for the usual crowd. A dozen or so couples, mostly in their 30s, would bring drinks and a plate of food. There'd be relaxed conversation, flirtation, music for dancing.

By no means were these sex parties, but after 11 the lights would dim, men would slow-dance with other men's wives, and hands might be allowed to stray.

No doubt the sex would happen after, when couples aroused by the flirting were in their own bed, or maybe there was some discreet swapping of partners. Bob and Jan were our nearest neighbours. This night they stayed back when everyone had left, overtly to 'help gather up the plates and glasses' but really because we'd begun to swap with them a few weeks before. When the dishwasher was stacked Bob led my wife away upstairs and Jan disappeared to the bathroom.

I undressed and sat slumped on a low sofa.

It's relevant to what happened to say that my length is a little more than average size. At that moment, sitting alone before anything had happened, it stood erect and throbbing just with expectation.

When Jan came into the room she was already naked. She was and is simply the most beautiful woman I ever met. That night her hair was pinned up, giving her a tall elegant look. Her body was all graceful curves, neither plump nor bony. Just the wide black triangle of pubic hair showed her animal-real, not a Greek goddess. She came over to me with a broad smile and knelt on the sofa, her two knees astride my hips.

As she lowered herself, she pressed my length down onto my thighs and rubbed her softness wet against the shaft. Our faces were a foot apart, looking into each other's eyes. I fondled her swaying breasts, she stroked my neck and lent forward to kiss. The kissing became more passionate, I kneaded her breasts, squeezed the nipples, reached under her to lift my length into her. For a minute she tantalised me, still rubbing hard against the flat of my shaft.

Then she lifted enough to let me pull it up and in . . . to the hilt, her hips still writhing against me. I squeezed the cheeks of her arse and ran my forefinger between them, meaning to finger-play her arse the way I knew she liked.

Her intent look melted into a wonderful, mischievous smile as she read the astonished look on my face. To my amazment, my finger had met the base of my length: I had entered all the way up her arse, so easily that I hadn't realised it wasn't her cunt. The revelation of her body's secret made me wild with excitement.

I lifted her off me onto the sofa and played her both ways in a frenzy of desire. In all our love-making that night, and in all the nights and days that came after, I could take her as easily the one way as the other.