I think this was prompted earlier somewhere, but I can't find it now.

John and Sherlock get together, are having sex, all seems fine. Sherlock eventually reveals or whatever some sort of daddy!kink, which John is pretty ok with. It's hot as hell and if Sherlock wants to be called a "dirty boy" while being fucked, who's he to complain?

Then, somehow, he finds out that Sherlock's father sexually abused him.