Hi Chris,
Of course it hurts, we're human... and we feel the loss of the break up and we're jealous of the new person with our ex partner. That's only human. And our ego tries to control them, so if they try to date a friend of ours, we try to put pressure on the friend to not date them.
But honestly, who does that help? Nobody. It's just an ego-serving act, based in jealousy. Again, only human. LOL
But once you're no longer together, you got no claim on who she dates. After all, she's going date someone, and so is he... why not date people you already know?
No it's not "wrong" for them to date. It's just hard for our human egos to accept the new relationship because of unresolved internal issues.
However, your friend flirting with your girlfriend while you were still with her is NOT cool.