Masturbation
As the video showed John beginning to pump himself faster, Sherlock's hand began to press on his own member harder. The dark-haired man decided it was time to take his member out of his pants and give it the proper attention it deserved.
The moment Sherlock had his cock in hand, he moaned. He was hard. Extremely hard. And it was all because of John. The detective looked at the screen to see the man pumping insanely fast, his head lain back on his pillow and his other hand grasping the sheets on his side. Sherlock whimpered at the sight and his member gave an unforgiving throb in his hand. He began to pump in slow, languorous movements, taking care to not pass his fingers over the sensitive head.
His concentration was shot when he heard John coming down from his room. Sherlock had forgotten that the video he was watching wasn't in real time and decided it was best to try and finish before John became curious as to what he was up to. Sherlock gripped his member tighter and began to move his hand up and down quicker than before. Still watching the video, Sherlock moaned at the feel of his hand touching his cock and his eyes latched onto the screen, getting hotter at the sight of John doing the same.
The dark-haired man could feel the heat building in his lower stomach, a tell-tale sign that he was nearing his end. Sherlock whimpered again as he accidentally brushed the head of his member with his slightly slick fingers. He didn't have any lubricant, but his pre-cum and sweat worked just fine.
Too focused on trying to keep himself going until the end of the video, Sherlock didn't even hear John in the kitchen making tea. He also didn't even realize he was moaning. And not only moaning, but moaning John's name, and rather loudly. Sherlock could feel the pressure building in his member and the heat become all too intense in his belly as John was nearing his end in the video as well.
Sherlock pumped hard and fast as he spilled out on his hand with a loud cry. Spent, sweaty, and slightly shaking, Sherlock laid his head back on the pillows and stayed there for a few minutes, panting hard. He jumped when he heard a knock on his door and went to grab his shirt and pants. He was too slow, though, as John opened the door and asked "Sherlock? You all righ-"
John stopped mid-sentence as he beheld his flatmate's pale skin and naked form with cum on his hands and stomach. He closed his mouth and was just about to leave without another word until his eyes landed on the man's laptop, which was still open. John furrowed his brows as he saw what he thought was his own room. Now, the doctor didn't have to be Sherlock Holmes to figure out what had just occurred, but John didn't want to believe it. Had Sherlock really just had a wank… while he had watched John have one as well?
Sherlock was stock still, his eyes wide and fearful and his mouth open. "John, I can- it was- it's not what you think."
"Is that my room?" John asked in disbelief. "Did you just- were you- is there a camera in my room?" He asked with incredulity.
Sherlock only shook his head slightly and wasn't able to explain anything as John bolted for the stairs, taking two at a time to get to his room. He had seen the angle at which the video was recording and no doubt had an idea of where the camera was. Sherlock quickly cleaned himself up with his sheets and yanked on his shirt, pants, and trousers before shutting the laptop and trying to decide where to hide it.
He heard John coming down the stairs quickly and cursed himself for not being careful enough. He gave up trying to hide the laptop, as John had all the evidence needed to make his own deductions on what just happened and he braced himself for the inevitable conversation to come.