Hurt and confused would be my first reaction. Why was this information not shared with you sooner? I would also be disappointed in those who knew this and did not share, as it is something that you should have known long before. What if there had been medical history related to your biological father that you would need to know? You should have been given the opportunity to know your biological father, as well. I'm sure that there was a reason that the information was denied you, but at the same time, you should have been allowed the opportunity to know the truth and make your own decision about your biological father.
After the hurt passes, however, I would realize that my father is the one who taught me softball, took me to girl scouts, and surprised me in Kindergarten when he returned from Saudi Arabia. My father is the one who taught me to cook, pushed me to succeed, and threatened my boyfriends when he thought they weren't good enough. I don't need to share blood with the man that has always been there for me, and just because another man is my biological father, doesn't change the relationship I have with the one that was by my side.
As a side note, I have 2 sons, one who is biologically mine, and one that is biologically my husbands. At the moment, they only know us as being their parents, and don't realize that they are not biologically related. They aren't old enough to understand, but when they are, we will sit them down and explain to them about their biological parents who are not in their lives. We want them to have the opportunity to seek them out, if they want, and ask them the questions that they are bound to have. Although it will be difficult for us, as well as for them, this is not something that should be taken away from them. It should be their decision, not ours. Just as it should have been your decision, not your parents. I hope that you still have the opportunity to meet your biological father, and answer those questions.