I'm going to talk about my best friend with her ex behind her back
Why are you hiding from your friend  that you'll talk to her ex?
Because it's none of her concern what I have to say to him.
That may be true, but shouldn't you be supporting your friend? Hiding things and talking to her ex doesn't seem very friendly.
But I'm mad and hurt because he never wanted to have sex with me. Don't I have the right to stand up to her.
You shouldn't pursue a friend's ex without talking to your friend about it. It's dishonest and disrespectful and can damage your relationship. Do you really feel like pursuing your friend's ex is worth that?