Maybe jealous isn't the right word.but if your girlfriend is ok sharing you with another girl, you can guarantee she'll expect you to have the same "ok" attitude if she's talking to another guy "friend".
My father taught me to be nice first because you can always be mean later. But once you've been mean to someone, they won't believe the nice anymore. So, be nice; be nice until it's time to stop being nice, then destroy them.
"Fuck excuses, learn to admit when you fuck up." This includes trusting the wrong people (and subjecting yourself to abuse). If you never admit you're involved, the same shit will happen to you, ad infinitum.