I think ATJC's suggestion was to look / dig deeper. Your OP talks about self-punishment. Catholicism has a lot of rhetoric about punishment and self-punishment which you could have absorbed when you were little, without any specific reference to masturbation, and then - not consciously though - applied to the act of masturbating when you became older. ATJC suggested that you look at "subconscious guilt" - simply not being Gung ho about religion now is not sufficient if there was some subconscious guilt planted in you many years ago.