My fiance kissed someone else, we still love each other, what do I do?

My fiance was in a dark place. We have had an amazing relationship for 3 years, we were about to get engaged and move in together. We have pets and he has a son who i became close to and we became a family. He was feeling all bad about himself (boo hoo), made buddy buddy with some girl at his new work, who may or may not have known about me, and they kissed. Twice! Two different times. I actually caught him out with her when he told me he was at work. (This period of him acting weird and us not talking lasted about two weeks) After I caught him I stopped talking to him and now he is begging for me back. He has made some huge changes, is willing to do anything I ask including getting rid of his new car (Where I saw them) and I still love him and know he is being sincere. I am sure we could have a very happy and faithful life together but now my family and friends hate him. I have witnessed some people hit rock bottom and become better for it and I have seen relationships grow stronger and more honest after such events. I believe we can be one of these relationships but I'm still kind of pissed and I hate that I can't talk to my friends or family. Anyone else go through something like this???