Again! He will be her step father and I am not messing with her head at all by having her call him daddy. I honestly dont know how all of you grew but your dad is not necessarily your biological father but it is the person who raised you and provided for you. If that is how it is than both my parents are really not my parents at all since I was adopted.
[/end lurk]
When you marry your fiance, how about you let your daughter know that he is her
step-father, and that he loves her and cares for her, and her biological father (I know y'all hate that term, but for lack of a better one in this instance...) is out there too. Whether that man chooses to be in her life or not, you cannot control, but you cannot deny that he DOES exist. And to do that, you would be lying to your child. I cannot fathom why any parent would want to do that.
Step-dad can love her and provide for her and be a father-figure to her, especially if the real dad decides to remain absent, but he can never be her actual father. And you only perpetuate the lie by allowing her to think that he is. It WILL hurt her later in life.
ETA - I think you should have more faith in your daughter. If your fiance really is the greatest thing to happen to this child, and if he continues to be, then her loyalties will lie with him. And think about how much resentment she'll have towards you when she finds out you lied (and she WILL eventually find out), not to mention the psychological damage. And she will be UBER pissed off that you didn't think she was smart enough to figure it all out. Trust is an important thing.
[/resume lurk]