Dear Boyfriends,
Just because a girl is insecure about you texting another female, that does not make it ok for you to tell them to fuck off. Or even tell them that /she/ says this. If you're aware she's going to have a problem with you texting someone at all, there's a much easier way.
"Hey sorry, I'm with my girlfriend, I can't talk right now."
See how nice and clean that is?
Sincerely,
I can't be your friend because she scares me