my life is just a collection of poorly made decisions with alternative music playing in the background
Telling your son not to “be such a girl” lets his sister who overhears the conversation know that being a girl is not a good thing and she should be sorry and ashamed of herself.
It also reminds your son that being a boy is better than being a girl and therefore he is better than any girl he will ever meet.
In russian if you cry you get called a cabbage. Vote to change “dont be such a girl to “dont be such a cabbage” say I.