Michelle, I do believe that ultimatums don't work in general. The OP's boyfriend has not cheated yet. He didn't have sex or had any physical intimacy with that girl yet (or that we know so far). Interpreting it as cheating is overreacting. I wouldn't brand it as emotional cheating either because they haven't talked about having sex. I am a firm believer that if anyone has been cheated on, the person should end all relations immediately and don't even give the cheater the time of day (for an ultimatum).