Somehow, I can just picture a perfectly asexual, no-interest-in-performing-the-deed-himself virgin Sherlock busting in the door every time John brings home a man/woman, and wanting to watch in the name of "research" (Can't you just hear "But, Jaaawn! It is in the name of /science/"). Sherlock is so determined to conduct his study that he won't even allow John to wank off undisturbed (I'll leave precisely how Sherlock seems to know exactly when John is about to masturbate up to the authors...discretion. Soyez creatif!).

Cue sexual frustration so intense that John in full BAMF mode breaks into Mycroft's office and demands that he keep his brother entertained for at least an hour so he can get a leg up before he spontaneously explodes in sexual frustration. Mycroft, who has his own mad teenage girl crush on the adorably handsome Captain J. H. Watson, offers to lend a helping...hand (or mouth, or other parts of the body. Really, I'm not picky...)

I'll love you, prompt filler, to the end of time if you can make this a Johncroft fic (there aren't enough in the fandom), but I will still make regular ritual sacrifices of plot bunnies in your name if it ends up as a Mystrade or Mollcroft fic...