Ok, most guys cheat on their gfs and dont tell them. You should be happy, that not only did he resisted the temptation, because he loves you, but also that he is honest with you. of course, you need to talk about it and find out how to deal with him feeling weak. He wants you to make him feel special (Not doing anything wrong) but showing him affection and apreciation.
You simply forgive them and be happy that they never cheated on you. Ask them what could have made them cheat on you and then try to work on the issue. Let them know that you are what they need and there is no need to look outside.