I know this may be hard to hear but I would leave her. What is the point of being with someone who has the desire to talk sexual to another man? It is hard to walk away but I am sure if it is happening now it is not the first time. People that are not happy with one person are usually never happy with one person. They always want more and more and the attention they receive is what makes them satisfied. If you want an honest real relationship maybe this girl needs to be reevaluated.