It was all because John's room had a leaky ceiling. Something to do with the skylight and the rubber stuff around the edge finally giving up. John had the attic room and it was a strange shape with sloping ceilings, so there was really only one place the bed could go. Right under the sky light. Right under the leaky bit. And John just couldn't sleep with rain dripping on him.
He had decided he was going to sleep on the sofa. Only Sherlock was still conducting experiments in the kitchen. So it was with a fit of generosity brought on by not really paying attention that Sherlock had suggested that John sleep in his bed. Of course Sherlock being Sherlock had forgotten all about this.
John Watson usually slept naked in the hot weather. But out of courtesy for Sherlock's bed he was wearing pyjama trousers, with Dangermouse on them; he'd always liked the cartoons as a kid, and had recently gained a new admiration for Penfold. And he was fairly fast asleep when Sherlock got into bed with him. Naked.
Sherlock always slept naked. When he slept in his own bed. He had perfected the skill of removing all his clothes in two swift movements. Nought to naked in ten seconds.
Only something wasn't quite right now. Because at least half of his bed seemed to be filled with warm, stocky, semi conscious Doctor. He tried to push John over a bit. For such a small man he seemed to be taking up rather a lot of bed. And he seemed to sleep very soundly.
Sherlock clicked on the light and examined the evidence. The proximity to John's sleeping form seemed to be having a strange effect on his own body. All the evidence pointed to the obvious conclusion that he found John Watson sexually attractive. He pressed himself against John's body and was rewarded with the smaller man turning over in his sleep and pushing against Sherlock's thigh. Pushing a rather obvious and startlingly large erection against his thigh to be precise. It seemed rather unlikely but on closer inspection John was rather well endowed.
Sherlock ran a hand down John's chest and belly, causing him to squirm in his sleep and grind a little closer to Sherlock's body.
"Mmmmm!" John murmured a noise of extreme satisfaction as he awoke and then he froze. Realising that he was currently pressed up to his flatmate. His rather brilliant, tall, handsome, enigmatic flatmate. "Sherlock what are you doing?"
"Well you see John. I have eliminated the impossible, and I know it may seem improbable, but I think I rather want to have sex with you."
John lay very still for a moment. Sherlock closed his eyes, visibly flinching. And then with a smile he pushed his pyjama trousers off.
"Okay then. But I'm not sleeping on the wet bit."