Sure, OK. But why is "teenage girl" being used as a pejorative?

Is there something wrong with writing like a girl?

It isn't being used as a pejorative. But an adult person shouldn't write like an emotionally unstable teenager if they want to be taken seriously. Teenage girls have a particularly clichéd way of writing and communicating. That's it. Being emotionally unstable isn't a pejorative either; it's part of growing up. It's called adolescence, when the brain is in a particularly turbulent and dynamic part of development.

I can't remember exactly what the now flagged comment said, but there is something odd and affected about adopting the slang/jargon of people a generation younger than you.