Questions about example sentences with, and the definition and usage of "Guyyy"

A: For “Being a girl” someone could use this as an insult. “You’re being a girl” which they are saying the person is acting weak and frail. (Which women or girls are not, but it’s often used in this manner)
Being is a current state of action.
“You are being mean”

For “Be a girl” this one doesn’t sound very natural, but someone could say it as a way to tell someone to act more ‘lady-like’ or ‘feminine’.

“Be” in this context it’s more of a command, or a desired situation.
“You need to be more respectful”
“I want you to be on your best behavior when we go to the store.”