Asexual!Sherlock is experimenting with masturbation and receives an embarrassing injury. Maybe he got a vibrator stuck in him and it's still going on high, he tried using a sound and now for some reason he can't piss, he was humping something and got stuck in it, or he managed to fracture his penis (which, yes, is actually possible).
Just give me panicked Sherlock begging John not to make him go to A&E.