Have you thought of getting counseling? I think this is one of those fears that for you is deep-seated, and you probably need professional help to work your way through it. But it will be difficult to parent with another person if that idea is always in the back of your head. The truth is, none of us really know, we trust in large part because many of us were lucky enough not to have this be a part of our experience. You're not wrong for what you feel, but it is something that you need to work through to be a healthy parent.
To be honest i think that can go through most people's minds. My bf is the best dad in the world but i sometimes think would i know if anything was happening? I always said that i will tell my daughter and any other children that if anybody hurts her or tells her not to tell anybody that she must always tell me because i will make it better. Im 100% sure, like you, that nothing will ever happen but i can understand from you past why you would be worried. It might be best to get some councelling for it and just know that you will do the best for your child. Good luck x
I think you need to have a very seriois talk with your fiance and then you should concider getting counciling. When you have a fear it is common to have these kind of reactions but there is alwys help. I suggest talking to your fiance and considering counciling because if you have a girl you may resent the father and become untrusting toward him and that could ruin your relationship.