I don't mean "not mad" in the sense that he is quietly planning his revenge, but rather that he would be very calm and understanding about the situation and that he will love you the same.
Let's say you cheat on your boyfriend one time, and either you confess this to him at some point, or he finds out and brings it up in conversation. However, instead of being angry at you and starting an argument or demanding a breakup, be would be like: "You are very sexy and I am well aware that the guys are all over you. I know that you are young and you want to have fun, so I couldn't blame you for something you did in the heat of the moment. I love you a lot, and if you still love me the same, than there is no reason to be mad about. You don't need to hide this from me in the future."
Would this be a green flag or a red flag? Let's say you knew for a fact that he never cheated on you. Would you be this calm about it if he ever hooked up with another woman?
Let's say that you don't have an open relationship. Would you be more or less likely to hook up with other guys if you knew that his reaction would always be the same?