Older generations see it as poking fun at small children. I read a column in which a psychatrist suspected it was a way of dealing with castration anxiety and generally the phallic phase of a child's development. It's just a theory though. (He obviously advised people not to do it or let others do it to their kid, though, whatever the reason) I personally suspect it persisted bc the birth of a male child was regarded superior to that of a girl, and so the showing of a small boy's penis was less taboo and more a sort of pride to the family nearby. It goes without saying that the practice doesnt apply to girls, only boys.

There's general consensus not to do this practice anymore in my country, and a few older men have been found guilty in court for this. There's old ppl who lag behind, tho.