I think you should leave it alone since you slept together when you were single there really isn't any reason for you to feel guilty about doing it, you both were single and lonely. Marriage is about a new start forgetting all past indiscretions and starting anew with your husband. If you feel that guilty and feel the guilty will never go away then you need to talk to someone about that but I don't think it will do any good to tell you soon to be fiance' about it since it happened way before him and has he told you every partner he has ever had? Some things are better left unsaid so not to start problems where there doesn't need to be problems. We all do something that we regret and wish we could take it back but we can't and what good is it doing to keep thinking about it. Telling your partner will only make him suspicious of the two of you being around each other and make him have unwanted jealousy and if you have no feelings for this guy other than friendship then there is no reason to tell him. When you get married get married with a clean slate and let the past stay in the past and be happy. We are all human and err so don't beat yourself up with guilt.