Sherlock is a confirmed Asexual but his logical side tells him that the ends justify the means and he has sex with someone ( male or female) to gather information for a case... Who he otherwise would NEVER sleep with.

Afterwards Sherlock is upset by what he did and can't stop thinking about it. He is clearly quite psychologically disturbed by what happened and even more disturbed that it affected him at all ( he was expecting to be able to compartmentalise).

Maybe John finds out and is horrified/comforting.