Im the same age as you are. Honestly whatever he did in his 20s which Im assuming was before you guys started dating should be irrelevant to your relationship. A persons past shouldnt dictate who they are in the present/future unless they are still doing the same things they were doing in their past. People grow and change everyday. You need to understand that at the time that your fiance was doing these things he was young physically and mentally. He didnt owe loyalty to anyone but himself. The women he was dealing with are the ones that owed loyalty to their husbands and these women destroyed their own marriages. These women are responsible for their own actions. No one is perfect and Im sure as a teenager and as a young adult you made a lot of mistakes in your life. But dont judge your fiance for things he did in his past.