Girlfriend attracted to best friend?

My girlfriend and I have been dating for two years. We both love each other very much. But one day earlier in our relatiOnship after meeting my bestfriend she had a dream about him (bf) and since then she has been sexually attracted to him. I told her how muc it hurt me that she had these feelings and She said how much it hurts her because she loves me so much. I tried to understand and we moved on. The problem is tha now she releives her stress from the hurt it causes her to tr and joke about it. and lately she has been talking about him constantly wanting us all to hang out more and the jokes have picked up. I had told her a few times that the jokes hurt but she said it was the only way to deal with it. I don't know what to think. It seems to be hurting our sex life and I have started to become extremely jealous (don't show it). I would tell her again but she doesn't seem to get it. I don't know if it is actually something to worry about or not. Wha do I do? What does it all mean? Help??