John assumes Sherlock doesn't know anything about sex. He's shocked to find that Sherlock masturbates. He's even more shocked to find that Sherlock's primary masturbatory material is this Livejournal prompt list - he may even write a few fills himself (and/or get grossed out by Holmescest/Mystrade/etc?) Maybe he also keeps a Tumblr of particularly good porn of himself? Draws his own and gets all excited when people tell him his art is so realistic?

The crackier the better!