John walks in and sees Sherlock bent over, red cheeked, crying and swearing with Mycroft's fingers deep inside him, and encouraging Sherlock "almost there".

John is understandably furious, demands it to stop at once and how long has this been going on for.

But Sherlock claims it's not what John thinks and this is all very necessary. Only the reason - Mycroft sending Sherlock on a honeypot mission and this is just preparation to prevent worse damage.