I guess someone breaks into 221B to steal stuff or plot device and get into a scuffle with Sherlock. They strip him naked and bind and gag him to somewhere. They put a cock ring on him and fuck Sherlock roughly. Sherlock is bleeding and everything hurts, especially his balls. Before they leave, they shove a big vibrating dildo into his ass, secure it so it continuously hits Sherlock's prostate.

Sherlock is on the edge but can't come. John is horrified and rushes to his friend, takes out the bloodied dildo gently because any movement causes so much pain. And the cock ring.

But Sherlock is too strung out and stressed and pretty much has blue balls and it hurts. John has to help his friend relax enough to orgasm. And it's the most painful orgasm ever.

Afterwards, Sherlock refuses to tell John who it was.