It would be an understatement to say that John had not been a fan of the mirror at first. He considered himself quite open and adventurous when it came to sex, and of course there was not much that he could deny Sherlock even if he wanted to, but watching himself just...didn't entice him the way it might have when he was younger. Time was unkind to mortal men, after all, and his body, once soldier-solid and well-trained was now that of a civilian more used to cozy jumpers than army fatigues. It wasn't that he was dissatisfied with his body, so he was glad that Sherlock hadn't even raised that as a possibility. He knew that he was still reasonably attractive, if the female attention he still received was any indication, and he did just fine chasing Sherlock all over London. It was just...something that did not interest him. That Sherlock claimed it was "for an experiment" did not help matters.
Still, it had interested Sherlock, and John didn't actually feel strongly enough about the whole thing to start a fight, so they brought a mirror into the bedroom.
It was laughable at first: apparently, Sherlock had not expected such a rapid victory, so he was uncharacteristically unprepared. Rather than wait until the next night when one of them (likely John) could get the chance to go out to a shop to get a properly sized mirror for hanging or something, Sherlock had dashed downstairs to borrow Mrs. Hudson's antique full-length mirror. At the very least, it had its own stand, which meant that they would not have to mount it on a wall or (John shuddered) the ceiling, and the fact that it belonged to Mrs. Hudson meant that it would probably not become a permanent fixture in the bedroom.
It had been awkward at first, to say the least. John had tried to ignore it, which served only to ensure that his eyes could land nowhere else. He watched as he worked the buttons on his shirt, pleased to note that his scar wasn't horribly noticeable...from several feet away...in the dark. It was a start, anyway. As he evaluated himself in the mirror, Sherlock moved in behind him and slid one arm across his chest. The other hand dipped to press against him through his trousers, and then Sherlock's words were against his ear, more breath than voice: "I would be more than happy to blindfold you, if you thought it would make this easier." John had the opportunity to watch his own face, right up to the tips of his ears, go red. He shook his head and pushed Sherlock away, changing position so that they were face to face.
"Yeah, I think you'd like that just a bit too much. Now strip."
When Sherlock began to slowly undress himself, John started to see the appeal of the mirror. Not only was he there on the bed in front of him, backlit by the lights from the street spilling in through the window, but his form was reflected in the glass beside them. When he was finished, he moved back behind John and tugged the zipper of his trousers. Moving at a maddening pace, he slid them down over his hips and wrapped his elegant fingers around John's cock. John couldn't do anything but watch their reflections as Sherlock stroked him once, twice—firm enough to drive him crazy, but too slow to actually get him anywhere. He still couldn't look away from their reflections, except now it was out of sheer fascination: the new angle was interesting, even erotic, and being able to see so clearly what Sherlock was doing in addition to feeling it brought things to a new level of intensity.
He ventured a glance at Sherlock's reflection and saw that he was already watching him. His pale eyes had darkened with what could only be described as lust. He didn't look the way he usually did when he was in the middle of an experiment—there was no trace of the cool detachment he wore when staring at petri dishes full of mold spores. He would have laughed at the absurd thought of Sherlock's "science face" in their bed, but just then Sherlock realized that John was watching him and met his gaze through the mirror. His lips curled into that familiar smirk and all reasonable thought fled from John's mind.
Sherlock continued to stroke him, keeping his pace slow and steady. He guided him closer and closer to orgasm, but never actually allowed him to topple over that edge. Neither one of them looked away, not even when Sherlock lowered his head to sink his teeth into the skin of John's scarless shoulder. He grunted against the welcome pain while his hips strained forward into Sherlock's hand, and all but whimpered with frustration when the touch slipped away.
The spell was broken. John tore his gaze away from Sherlock's and turned around to press him into the mattress. He made quick work of Sherlock's pants (perhaps his wordless defiance of John's earlier command) and let them slip to the floor beside the bed, forgotten. He took him into his mouth without preamble, sucking deliberately and allowing Sherlock to fill his senses. Hands came down to grip in John's hair and he couldn't help smirking a bit with pride at the response, even though it happened nearly every time. It had taken some practice, but John knew by now exactly what worked on this man, what he liked and what was most likely to render him speechless. He let his eyes wander up the pale writhing body to Sherlock's face, which was turned toward the mirror. Though his eyes were narrowed with pleasure, John knew he was watching the two of them. For his benefit, John pulled back and allowed the cock to slip out of his mouth, then ran his tongue along its length from base to tip. Sherlock growled with pleasure and his fists tightened in John's hair, demanding, urging him on. He resisted.
Two could play that game, after all.
John worked one finger inside Sherlock as he continued to tease his cock, flicking his tongue along the tip but never closing his mouth around him the way he knew he needed him to. Only when the man beneath him was gripping at the sheets and hissing words that might have passed for pleas did John relent, and even that required separating from Sherlock for a moment as he retrieved the small tube from the bedside table. A quick application and he returned to his previous position, taking all of Sherlock's cock into his mouth once more and simultaneously sliding two fingers inside him. Sherlock swore under his breath, but the fact that it trailed off into another growl told John that it was good. Quite good, in fact. It was his turn now, to bring Sherlock close to the brink of orgasm as he added a third finger, and he held him there despite his own cock's objections. He loved feeling Sherlock come, cock throbbing in his mouth and muscles clenching around his fingers, but now was not the time. Their eyes met in the mirror again and John grinned again, twisting his fingers together inside Sherlock.
"John..."
As intriguing as it was to watch his otherwise somewhat-stoic lover squirm and plead, and as much as he enjoyed these fleeting moments of unfiltered, unrestrained humanity, that voice was John's undoing and they both knew it. He pulled back one final time and sat up. Another quick swipe of lube and a bit of fumbling with a condom and then he was pushing his way into Sherlock slowly—so slowly. Too slowly. When he was fully inside him he paused to push one of Sherlock's knees higher against his chest. Sherlock had finally torn his eyes away from the glass—now they were hidden behind tightly-closed eyelids. John reached down to thread his fingers through Sherlock's hair and tugged softly, eliciting tortuous moans from the man beneath him.
"John," Sherlock choked out. "If you don't start fucking me in the next ten seconds, I am going to flip you over and do it myself."
He believed him, and thus did not waste any time marveling at the interesting qualities the word "fuck" took on when uttered in Sherlock's posh accent. Instead, he began moving slowly, watching his face as he did. He tried to hold his pace steady, remain in control, but he was already so wound up that it seemed impossible. He began thrusting hard, driving deep, and both men realized that the time for teasing was over. Sherlock's arms were locked around his knees, holding his hips at exactly the right angle so that John hit his prostate with every movement. John supported himself with one arm while the other hand gripped Sherlock's hip to pull him even closer. His fingers bit into the pale flesh, and he knew that he'd leave marks, but that was hardly important at this point. What was important right now were the noises the Sherlock was making (desperate, mewling sounds that seemed both highly inappropriate for the man the rest of the world knew and perfectly appropriate for this side of him which only John had ever seen), the heat, and the friction between their bodies.
John circled Sherlock's cock with his hand now and stroked it with all the patience that he could not muster in the movement of his hips, coaxing him closer to coming, drawing the orgasm from his body. Without realizing it, he was chanting in Sherlock's ear, begging him to come, love, come with me, I'm ready to come, are you?
Sherlock's body responded and a moment later so did John's, and they came not simultaneously but near enough, gasps and moan filling the air of the room. John slipped away for a moment to take care of the pressing matter of the condom, and returned with a warm cloth for Sherlock. After they had cleaned off, they stretched out together in John's tangled sheets with Sherlock's back pressed against John's chest, and he found a new appreciation for the mirror.
If he lifted his head just a bit (or, say, propped it up with his hand and elbow), he could see Sherlock's face perfectly in the mirror. His hair was a mess—a proper mess now, not the contrived and fashionable mess that he preferred—and fell in dark waves around his face. His eyes were closed at first, but they opened almost sleepily when he felt John shift behind him. John grinned at him through the glass as they made contact. He traced lines down Sherlock's shoulder, along his arm and across his chest. He paused to lightly pinch a nipple and Sherlock actually laughed before lifting his arm to move John's hand. This was another side of Sherlock was John was reasonably sure he was the only one to ever have seen: this languorous, utterly satisfied side that always put John in mind of some sort of feline. He lowered his head to nip at the tender skin on Sherlock's neck. "Was the experiment a success, then?" He mumbled. Sherlock responded with a yawn before closing his eyes again.
"What experiment?" His voice was a little too innocent, a little too confused, and John just chuckled before pressing his nose against the place where Sherlock's neck met his shoulder. He breathed in his scent and exhaled peacefully. Maybe he would wait until tomorrow to tell Sherlock that they could get their own mirror.