What to do if your boyfriend cheats on you?

I just want some help so that I can take further action. My boyfriend has been cheating with his secretary. I caught them in the office and he fired her immediately. But I’d don’t think I can even tolerate to look at him and I’d like to leave him ASAP. He doesn’t want me to leave him and says it was just one time which I don’t believe. He wants to go to therapy and give our relationship another chance and I don’t.