John discovers fanfiction about him and Sherlock, and out of curiousity reads some. It's not long before he finds the R rated stuff. He knows he should be horrified that somebody is writing him and Sherlock in all these sexual sisuations, but he can't get he thoughts out of his head.

Soon, the thoughts of him and Sherlock start rulling his life, and he even gets hard just seeing Sherlock bent over looking for something.

Of course, it's not long before Sherlock discovers the cause, and seduces John why reading out the lines from one of John favorite sex scenes.

Preferable bottom!Sherlock, and bonus if you use the word plunge in there