My reaction would be more violent if my girlfriend was with another man.First i would give the man a serious beating. Second i will grab my girlfriend by the collar with all her dresses and her personal things, go with her till the exit door and say her:"You betrayed me and you lied me you bitch. You are definitely not more accepted in my home and i don't want to hear your lame shit excuses.Try to come back and you will regret it, understand. Now fuck off and play with your new friend." Then i would close the door violentely behind me.