I am not so sure that this mastrubation rumor really is a myth.
Many societies want to controle the behaviour people and especially children in all ways, and register everything about their behaviour.
Myself I live in a society with very strong tendencies in that way - Norway.
I remember when I was 7 years old, my parents got a large formula to fill out about everything they knew about me. They sat up a whole evening filling out that formula.
One of the questions were: "Does the child play with his genital". They read that question load, and therefore I remember it.
I think they answared no, even though they knew that I indeed had the habit of playing with may genitals.
I allways had to be nude at the school physicals, and they allway checked my rectal opening, but did not usually touch my genitals.
But when I was 12 years old I was called in to a more thoorugh exam of my pelvic area.
The schoole health service in Norway are very eager to call in for "special exams", or "targeted exams" as they are called officialy. Of these, psycho-social exams, intimate exams and sensorial exams seem to be the most common.