What should you do if your girlfriend flirts right in front of you and the guy flirts back?

This guy you like isn't yours until you get together. now, if the girl who is flirting with him knows that you like him, that is kinda rude, but you can't really do much about it, except for trying to make him like you. make your move first. until you two are going out, this other girl can do whatever. ask him out, or let him know you like him. if he goes for the other girl, its okay. stuff happens.