In case of women who are not married they may be denying a father but deliberate? Many men have no sperm, that is the case with my husband and that is the reason we adopted. We might have had an AI child in 1963 but my husband has an adopted brother and sister so that was the natural way for us to go.
Adoptions are more open now, in the past you were told nothing about the birth parents, NOTHING.
And that was WRONG, because it is deceptive. It is one of the worst lies and once a child catches you in this, and they eventually do, they never trust anything else you say to them again. My entire family fabricated a preposterous story about my birth father (BTW, I am the 2nd immaculate conception ;-). Even as in my earliest childhood, I recognized the fallacy in their story. It wasn't until I turned 21 that a cousin told me the truth. When I adopted my daughter, I made certain she understood she was adopted from the earliest years. On the other hand, I chose not to have an open adoption as I believe that creates confusion. My daughter chose to meet her birth mother and then chose to come home to her family.