Religous castration.

[An Onymus (imported)]

I have to think that autoeroticism came to be seen as "wrong," simply because people tend to do it furtively, and they do it furtively because it involves going out of control to some extent--the person doing that sort of thing, gets carried away by the excitement of the act. And people delight in making fun of someone who seems to have lost control of what he's doing. As far as I can tell, autoerotic activity doesn't do any harm to anybody.

It's possible that, as indicated in the story of Onan, folks in ancient times felt that those who did autoerotic things, would fail to reproduce normally--and reproduction was considered very important in an era when most tribes and clans were relatively small. People figured they needed all the babies they could produce, to keep from being overwhelmed by other groups around them.

Guilt over autoeroticism has probably been one of the most pervasive features of human psychology throughout history, and one of the most nonsensical.

Incidentally, to my knowledge, animals don't masturbate in the wild, although there is a rather humorous story about chimpanzees at a zoo in Europe which were taught to masturbate by a zookeeper.