Several times Sherlock has brought up knowledge about kink for their cases, so when they're checking a possible scenario for a crime they're working on, John goes with his instincts. 'wrestling' Sherlock, who's playing the baddie, becomes actually wrestling sherlock, right down to the ground, where John hoarsely kisses him and bites his lip- hard- and tells Sherlock to choose a safeword. To his surprise, Sherlock responds mildly that his knowledge of kink is only professional, and really, John, -insert sherlockian analysis here-.

Up to you how John reacts- is he okay with this, does he discuss kink with Sherlock to see if Sherlock might have an interest, is he embarrassed (maybe the start of some issues in their relationship, or a possible cause of ED?). If you want to go evil, maybe he shrugs and says "no safeword, then" and just keeps going.