I have a dead beat father, and it wasn't until I was 18 that I figured this out. I idealize my father. My mother and younger brother would hunt him down and clean him up just so he could spend my birthday with me because that is all I wanted. I was blind to what and who my father was/is.
Be honest with her, and tell her that fathers are made when children are born, but dad is a name earned. Tell her that her father right now is not ready to be a dad, and that if he changes his mind he has to earn it. Also let her know that you will explain more as she gets older. She is smarter than your seeing, and will need to be told more details as she gets older.
Also don't call him by the dad name, use his given name so she an make the distinction. This will also allow her to see that your SO is a dad not just ________ <-insert name here.
I know for years I wish my mother was more honest with me about my father.
I hope this helps..... Also remember if she is acting like this she is smart enough to understand the dynamics more that you think.
Mom to Breille (cheese) 9-14-00 & Mackenzie (Mac) 12-31-10
I do know how to spell, my smartphone does not.