The first time was an accident. John ended up walking home from the surgery in the pouring rain because he'd managed to forget his wallet at work and thus didn't have his Oyster card. When he got back to the flat, Sherlock was sulking dramatically on the sofa - even just two days of forced celibacy seemed to be putting him in a post-case-funk type of mood - and didn't even acknowledge his presence. John just shrugged and headed for the shower to warm up.

The problem came when he finished and realized that a) he'd neglected to bring down dry clothes to change into, and b) Sherlock was still haunting the living room. There was nothing for it but to wrap the towel around his waist and hope Sherlock wouldn't look up before he could escape up the stairs.

Sherlock noticed, of course. Actually, noticed might have been a bit of an understatement - Sherlock shot to his feet and charged to intercept him, stopping only an inch before he'd have plowed John over.

"You're being unfair."

John blinked. "I'm doing what now?"

"This." Sherlock's vague gesture encompassed John's bare torso and most of what was covered by the towel. "You're taunting me, and I hardly think that's reasonable given the rules of our wager."

"I'm not taunting you, I'm-" It took a moment for Sherlock's words to click in his head. "Wait - seriously?"

Sherlock's glare could have melted through steel. "What, playing innocent now? You don't just get to wander through the flat half-naked, flaunting everything. It's not fair."

Ooh, this is unexpected. "I don't recall saying anything about flaunting in our wager," John said slowly. "Pretty sure I would be well within my rights to wander around starkers if I wanted to. Hell, I could pull the same kind of shit you've been doing for months - flop on the sofa and pull myself off at two in the afternoon, just because. We didn't set any restrictions on my wanking, just yours."

Sherlock's eyes dilated so fast John would have sworn he was on something, if he hadn't just been acting so like himself moments before. "Yes."

"What?"

"Do it." He latched onto John's arm and started tugging him toward the sofa. "If I can't masturbate, at least I can watch."

"Christ, Sherlock!" John dug in his heels and tried (unsuccessfully) to wrench his arm out from his flatmate's grip. "That's not-"

"You didn't do it in the shower - I can tell. Not flushed enough, not long enough without the sound of the water changing as you shift position. You were cold and out of sorts; it didn't occur to you to bother. I, on the other hand, have been thinking about it all day long. It's only courteous."

"I - it's not - fuck. There's nothing courteous about demanding your flatmate beat off in front of you, you know!"

"I know, but it seemed like the argument least likely to make you mad. Please, John." Sherlock's voice held a hint of desperation. "Would it help if I turned my back and just listened? I'll do that, if it makes you feel better."

"What would make me feel better would be for you to leave off and let me go hide upstairs and die of mortification."

"Oh, dull." Sherlock waved off his objection with his free hand and tugged on John's arm again with the other. John had to choose between overbalancing face-first onto the sofa or letting the towel fall off. He picked the former.

"You know I'd deduce what you were doing up there anyway, so why not just do it here?" Sherlock whined.

"Because I'm not a bloody exhibitionist?" John snapped back.

"Not yet, maybe," Sherlock countered. "But you could be. Picture it - I'm sprawled here in my chair, desperate to be touching my cock but I can't." The sound of creaking leather coming from somewhere in the middle of the room, out of John's field of vision, confirmed that Sherlock had indeed just flopped into his armchair. "You, on the other hand, are stretched out on the sofa and if you just pull the towel away, you're perfectly free to enjoy yourself. Roll over - you can watch my back, at least, even if I can't watch you. You can confirm I'm not peeking."

John did roll over - keeping the towel wrapped tightly around his waist as he did so - and yes, Sherlock was facing the other way. And now had his shirt off, so his bony bare shoulders were clearly visible over the arm of the chair. John couldn't see his hands, but from the way Sherlock was squirming, it looked like - yes, there went his trousers too, sailing across the room into a messy heap against the far wall. Sherlock was in just his pants and it's not like John hadn't seen it before - Christ, he'd seen way too much of his flatmate recently - but this time was completely different because Sherlock's razor-sharp focus was entirely on John despite his dramatic posing.

"No reflective surfaces, either, if that's what you're worried about."

He hadn't been, but the idea probably would have occurred to him halfway through his wank. Christ. He wasn't actually considering this, was he? One part of John's brain was two seconds away from saying screw this and stalking upstairs, but a much larger part finally got his conscious attention by pointing out that yes, he was considering this, and in fact already had the towel lying loose over his hips and one hand inside it, fondling himself gently.

"Why?" His voice came out a bit squeaky, which Sherlock would definitely notice, but it was too late to take it back now.

Sherlock just shifted in his chair and shrugged, that maddening if-you-can't-figure-it-out-yourself-it's-not-worth-my-time-to-tell-you half-shrug he usually used at crime scenes when Lestrade asked something particularly inane. "Because you turn me on." He hmmed a bit and wriggled deeper into the cushions. "Do that thing with your wrist and your thumb - you make the best noises when you do that."

"I don't even want to know how you know my technique."

Sherlock tossed his head back, and John knew he was rolling his eyes. "No reflective surfaces here, John. There's a mirror on the right door of your wardrobe, though, and you frequently leave it at the correct angle to reflect your bed from the hallway."

". . . That would mean you stand at the top of the stairs?"

"Well it's not like I'd barge in on you - that would be rude. Ooh, your breathing just changed. Do whatever that was again."

John groaned. "It changed because I'm angry. Spying on me when I think I'm alone is very definitely Not Good, Sherlock." He sat up and pulled the towel back tight around his hips.

Sherlock turned at the noise, his eyebrows drawn together in consternation. "How is me watching any different than me listening?"

"Yeah, you ponder that. Let me know when you figure it out." John mustered as much dignity as he could while still only half-covered by a towel and stomped his way up the stairs.

I wasn't watching for sexual pleasure; I was just curious about your technique. Not attempting to "get off." -SH

Never had the opportunity to compare my own masturbatory routine with someone else's and I didn't realize you'd mind. -SH

Didn't tell you I was there because I didn't want to bias the results. -SH

Upon further reflection, I've realized your anger is probably a reaction I should have predicted, given you didn't know my motives were purely scientific. -SH

I'm sorry. -SH

I'll go out for a walk so you can get some privacy to deal with that erection. -SH

I'm bloody hard. -SH

John snorted at his phone and set it back down on the bed beside him. The mental picture of Sherlock trying to go for a walk while painfully aroused went a good way toward soothing his annoyance at his irritating, socially stunted flatmate.