It's a cold, winter's night in England. The kids are asleep, my husband is snoring quietly beside me, but I can't sleep. It's nearly midnight. I love my husband, but I am about to betray him. Again.

I slip quietly out of bed, tiptoe downstairs, and go into the dining room. The straight-backed chair is cold and hard, but I flick up my nightie so that I can feel the cool wood against my buttocks and thighs. I start up my laptop, and as I wait for it to boot, in the soft glow of the screen, I see the goosebumps on my arms, the shadow of my nipples as they press against the thin, silky material of my nightdress. It is cold, but I don't want to put a heater on, it is as if I am punishing myself with the cold. A shiver runs through me, but I know it's not just because of the icy feel to the room.

I navigate to an obscure directory, click on a file with an unremarkable name, and enter my password. The presentation opens with a photograph along a deserted beach, the impossibly white sand bleached by the strong sunlight. I click the mouse, and the picture changes to one across and down the beach, looking out at the blue sea and white breakers. In the middle distance stands a post buried in the beach, the remnant of some ancient jetty, the wood smoothed by the action of the sea, and turned silver by the salt. On either side of the stake can be seen small protrusions which, on closer inspection, turn out to be parts of a body seen from behind. A female body.  My body.

I think back to how I came to be on that beach. It had started innocently enough – he had posted photos on Deviant Art, not 'dirty' pictures, but landscapes, cityscapes, all kinds of things. I had commented that I liked his style, an email conversation started, all very innocent. But he had checked out my stories on the site, seen that I was into bondage in a big way, and had emailed me a proposition: 'I would like to offer my services as a photographer for any BDSM fantasies you might like to enjoy…'

Of course, I laughed it off – I'm happily married with two kids, I wouldn't, couldn't possibly do that! The conversation went on, the subject was dropped.

If only he'd said 'I'd like to photograph you naked, tied up', I would have just dismissed it as the usual – some 'pervert' trying it on. But he professed no interest in bondage,, and I believed him. He had no nude pictures on the site, not even any clothed portraits. But what really wormed its way into my consciousness was that phrase, 'any BDSM fantasies you might like to enjoy…' I had spent years translating the images in my head into stories I could read, somehow making them more real. Posting them for others seemed to legitimise them further. But to act out those fantasies and have a permanent record of it… the thought ate away at my soul for months, and I knew, somehow, I had to make it happen. I click the mouse.

The picture is of me, kneeling on the sand, arms above my head. I am wearing a bikini bottom and a T-shirt which don't meet, exposing my midriff. My legs are slightly open, so that my feet are on either side of the post. My arms are held above my head by the rope around my wrists, tied higher up the post. I remember the feel of that rope, coarse enough to catch my skin, rubbing it until slight burns can be seen. I have already tugged on that rope as hard as I can, wanting to convince myself it is secure, unbreakable. At last.

But my attention soon switches to my mouth, my lips stretched tight around the bright red ballgag filling it. I can still feel the smoothness of its surface as I explore it with the tip of my tongue, swirling in the small lake of saliva built up in my mouth behind the silicone dam.

The photograph captures perfectly the tensions in my body. The tightness of the hamstrings in my thighs, lightly dusted with grains of sand. The twist of my trunk as I writhe against the wood. The flush on my cheeks, as my hair blows gently across my face in the sea breeze. But, most of all, the look in my eyes – some fear, plenty of excitement, but mainly raw, wanton lust. All those years of longing, finally being satisfied. As I click the mouse again, my other hand slides down between my thighs…

This one is very similar, except that the T-shirt has been ripped open, exposing one of my breasts. Everything that happened that day had been agreed beforehand, and would follow the path I had laid out to him. Thoughts of betrayal, of being abused while tied up, wild ideas of being sold into slavery, swam in my fevered brain, struggling and finally drowning in a sea of adrenalin and arousal.

There was to be a safe signal, in case I felt nervous or uncomfortable, and wanted to stop – both hands spread out wide, fingers pointing to the cloudless sky. I had told him I might be screaming, begging him to let me go, but unless my hands were open, I wanted him to carry on. He looked doubtful, nervous, but he agreed. I smile slightly as I look at my clenched fists in the photo. The scream I was emitting at the moment the shutter fired had forced some of the saliva past the ball, and there it is on my bottom lip, running down to my chin.

The next slide is one of my favourites. The T-shirt has gone completely now, replaced by ropes around my body which frame and crush my large breasts. I hate looking at my breasts – after childbirth and breast-feeding, they are too big and too saggy to be seen naked and unsupported. But here, with the rope criss-crossing them, they seem to be no longer objects of repulsion and ridicule, but emblems of my helplessness and submission to the ropes. They look… beautiful.

I hurry on. Now, the bikini bottom has been cut away, and a rope has been tied around my waist and between my legs, the classic crotch rope. It helps define my soft waist, the still slightly flabby belly held in by the tight band, cutting into my sides. The rope, descending vertically, disappears into the depths of the forest of pubic hair, with another seeming to ascend alongside it, on the return journey, to a knot tied near my belly  button. I remember urging him to pull it 'tighter! tighter!', the words garbled by the gag, the spume bubbling on my lips. The feel of the rough cord digging into me between my plump lips, crushing my aching clitoris, is etched on my memory. My finger follows the trail of that rope, and emerges wet.

The next slide shows the first waves lapping at my knees, taking me back to that long, agonising wait for the tide to bring the sea back to me. A close-up on my face, damp with sweat,  plastering my hair to my forehead and holding a few grains of sand blown on the breeze, shows what appears to be a look of fear in my eyes, but I know why it was there – I had been fantasising about how I came to be in this seemingly perilous position. In my imagination, my husband had fallen in with the mob, now they had abducted me and left me here to die, unless he could pay the extortion money in time. Yes, it was unoriginal and faintly ludicrous, but to my lust-crazed mind, it was sufficient justification for the scenario I had requested.

I click again, and now the waves are breaking over my thighs. I was deep in the fantasy by then, eyes closed, not seeing the camera or the photographer, it was just me in the bubble of fantasy, about to drown in the cold sea, tied to that post. There is a photograph of the waves breaking against my stomach, the spray falling across my bound breasts, but then he couldn't take any more, he had cut me down and dragged me out of the surf and up the beach, where I fell onto the hot dry sand. My hands were still clenched into tight fists…

There is one more picture in the set. The ropes have gone, the gag removed. I am lying on the hot sand, on my back, legs apart, fingers desperately working. My face is twisted in a rictus of lust, as I bring myself to the climax I so desperately needed, my mind full of the sounds and sensations I had experienced only minutes before. My hand is shaking as I close the presentation. With barely a pause, I open the second file.