My Mom totally thought this way and even won, although the man was my stepfather.
I never met my real father and my stepfather came into the picture when I was super young so I thought he -was- my father. My Mom told me to call him Dad and lied to me and everything.
Later they divorced and I couldn't understand why he never so cold and distant to me but not my Sister. Found out that he wasn't my Dad and was bitter because the judge who handled their divorce forces him to take weekend custody of me.
It was awful. I would've rather been told that he wasn't my Father than have to go through the experience of thinking my Dad hated me like that.