I stand before you, the jury, accused of a heinous crime, and extend my apologies in advance if this account appears somewhat graphic. But I assure you the level of detail is necessary to provide context that will allow you to reach your verdict.

First of all, yes, I did it. There's no escaping the fact. Nothing can undo the wrongs I have committed and there's little excuse for my actions, beyond raw rage. But I ask you to consider what you would do at discovering the person you married – the person you had loved and cherished for twenty-five years and bore your children – committing adultery in your own home.

At first, I thought Lauren was alone with her toys. We both have a healthy sexual appetite, and it wouldn't be the first time I'd come home early to find her already in bed, awaiting me. She often starts alone.

As I approached the ajar bedroom door, her panting spilled loudly from the other side. A grin spread across my face as I imagined her on all fours, curvy rear upturned, hand between her legs, pleasuring herself, thinking of me. I love to listen as her excitement soars so I stayed outside, the intensity of her moans increasing.

With the expectation of walking in on her and getting straight down to business, I rearranged my rising erection so it was pointing straight up in my trousers, already straining against the material. As it grew harder still at the sounds of her impending orgasm I unbuckled my trousers and allowed them to fall to the floor, stepping from them.

My wife is beautiful. A green-eyed brunette with a trim, shapely figure to which time has been kind. I appreciate no body is perfect, but hers excites me beyond measure. As her staccato gasps reached their crescendo, I knew from experience she was close. Our youngest had recently left home so there'd been no reason to hold back any more, and we had taken full advantage of every surface, horizontal and vertical, as we rekindled the intensity of our courting twenties.

Yes, life was good. Or so I thought. The next sound I heard sowed seeds of doubt.

A sharp spank rang out and Lauren gasped, then groaned as the heat spread through her body. She loves to be spanked. It makes her horny and very wet. The real problem was the next word she uttered:

"Again!"

Another crack rang out and she groaned louder. "Fuck yes! Harder!"

The next was accompanied by a male voice. "Oh you love that, you dirty bitch."

My mouth went dry. In a classic case of denial I tried to convince myself it was the TV; that she'd put on some porn to get herself in the mood. But part of me knew that wasn't the case.

It's difficult to describe the feeling when someone you thought you knew so well betrays you, but I'll try. I felt empty. A hollow husk of myself, along with a draining, cavernous inadequacy.

What was it I couldn't give her that she needed from someone else? All she had to do was ask and I'd have willingly delivered. Anything.

Jealousy then raced through me, a dark virus unfurling in my veins. So much of it infected my system at once I had to steady myself against the wall of the corridor outside the room. It wasn't rage, not then. Rage came later. It was disbelief, common sense refusing to accept what I knew to be true. I didn't want to know, oblivion more attractive than insight. But like a perverse case of Schadenfreude, I had to find out.

Forgetting my state of undress I gently nudged the door to see them reflected sidelong in the dresser. Lauren on her knees, face thrashing into the pillow, his hands gripping her hips while slamming into her from behind. I stood agape, pretending it was someone else's wife being ravaged by a man she had the audacity to call 'just a friend'. Paul Wilson. Colleague. Balding slimeball.

He spanked her again and I blinked, recoiling slightly, what little saliva remained in my mouth desiccating my throat at her resulting sexy sigh. I couldn't move. Looked down to see if my feet were still attached to my body, surprised to see my erection full in my boxers. What kind of freak does watching adultery excite? Watching his wife cry in ecstasy as her orgasm crashes through her stiffened body, another man plundering her soaked entrance, calling her filthy names and slapping her reddened behind. It made me doubt myself as a man.

But when she did that all-over body shudder – the one I naïvely assumed was reserved for me – flashes of our times on the kitchen table, the sofa in front of a crappy B-movie, the back seat of the Audi, and over the plastic patio furniture, stung my mind. I remained hard at the visions, painful as they were, straining, excited, inexplicably bursting to come. Lifelong promises shattered, yet throbbing, aching with need for her one last time despite her infidelity.

Even when she begged for him to fill her, and I quote, "Slutty wet cunt," I didn't flag. Squeezing my eyes shut in a vain attempt to stem the tears, I heard him roar and spank her mewing form, unleashing the same white-hot lust into her body as I simultaneously fired into my underwear, feeling it seep through the fabric, hot and sticky against my groin.

Only when he asked if she loved him and she purred, "Yes," did the rage hit. Blind rage, it's called. An out-of-body experience I found impossible to prevent, charging into the room amid the shrieks, begged reasoning and false attempts at covering modesty, long since evaporated. Boiling red rage, the same colour as the blossoming stain on the bedsheets.

Yes, I did it. I'm not proud. He'll live, but I have to live with myself. It's up to you how.