Sherlock has to watch some gay porn for a case. He's not expecting it to have an effect on him, but he starts imagining John doing these things to him and has to take a few minutes to calm down. He goes about the rest of his day determined to focus on the case, but John keeps brushing against him or touching him or just getting close enough for Sherlock to smell him, and Sherlock is getting increasingly aroused and frustrated as the day wears on.

It ends in enthusiastic, rough sex. (Bonus points if it's somewhere inappropriate.)