I've seen fics where John is insecure about his bullet wound, or Sherlock is hiding self harm scars, and they all tend to end with the other person kissing the scars and the person with the scars being fine with that. What I have yet to see is a fic where the person with the scar does NOT want it touched at all. It isn't because they think it makes them ugly or a bad person, it's because they don't like the sensation of being touched there.

TLDR: Sherlock doesn't want his self harm scars touched/John doesn't want his bullet wound touched.