Deep within Sherlock's mind palace, there was a room. It was sealed away with a shiny red lock. Sherlock couldn't even get in unless he really, truly wanted to.
Open the door and smoke fills the vision, smoke from cigarettes and pipes and cherry flavored cigars. It clears away after a few moments, but the red tinged room remains hazy. Along the walls are shelves and cases that hold black leather instruments that can really create magic if used properly and with just the right hand.
The person in the handcuffs suspended from the ceiling, more often than not anymore, was John. John with his battle scars and grey whiskers and nice round bum. John who would groan and wince and beg for mercy when Sherlock used the bullwhip or the paddle that left a lovely cursive 'S' surrounded by hot red flesh.
That isn't to say that John is the only one who's been whipped and gagged to trembling ecstasy. Certainly not. There was Lestrade, Anderson at a point. Even Irene a few times.
Oh Irene was far trickier, far tougher to mold. She would smirk unless he found just the right spot, and if he took too long in finding it she would twist and navigate her way out of the cuffs before disappearing much like the Cheshire Cat, fading away until her raspberry smile was all that was left.
Sometimes, she'd even change the rules. Sometimes he would be the one with his arms tugged upward and his body pale and defenseless. She would whip him into a frenzy before walking away with some female on her arm. She would leave him there until he could find a key and free himself.
Sure, John was married and Irene was nowhere to be found. Lestrade was dating and Anderson was, well, Anderson. But in the mind palace, with it's labyrinth of doors and invisible gilded walls, the rules didn't apply.
After all, a little imagination never hurt anybody.