I've seen a lot of fics where John dates another man (Lestrade, Mycroft, OMC...) just to make Sherlock jealous. I can't believe that Sherlock wouldn't see through this immediately. So: Sherlock realizes exactly what John is doing and decides to play along--call it an experiment. Frustratingly, he finds that even knowing exactly what's going on, he can't control his instinctive reaction to seeing John kissing someone else.