To understand how this ‘masturbation is sinful’ idea came to be part of Christianity even up to today, you have to understand some history. So read on. This is fascinating and scary stuff.
As for masturbation, this was never mentioned, nor was it ever listed as a sin in the horrendously detailed lists of sins in books like Leviticus. The Jews clearly regarded masturbation as a natural fact of life and not even worth a mention.
Not only did the Jewish Bible never mention masturbation, neither did Jesus or any of the Apostles. There were of course warnings against the misuse of God’s gift of sexuality, but masturbation was never included or named as one of these ‘misuses’. GO