You are making a case that your progenitors may not have in fact, partaken of the tree of knowledge.
Obviously, the story of Adam and Eve requires some interpretation on the part of the reader. If Adam existed in the world before Eve, Adam would have had no need of any sexuality. It is more reasonable to interpret that in creating Eve, God created male and female sexuality in humans at the same time, from the same materials. In that light, the creation of Eve is not derivative of Adam, but rather is a redesign of the human species for sexual reproduction. The rib is only symbolic of the materials used to create humankind. (Of course, men and women, biologically, have the same number of ribs.)
As to the generations that followed, Eve had 3 sons: Cain, Abel, and Seth. While Cain fathered Enoch, that line perished. Seth is said to have fathered many surviving offspring. Seth is said to have married his sister, Of course, human genetics have changed greatly over the centuries. Perhaps birth defects were not so prevelant among children born of closely related parents at that time. Not to mention the whole "God overseeing the whole operation" aspect! In any event, if you can believe in a creator, surely it is not much more to believe that the original genetics designed by the creator could allow for successful procreation among the closely related. Noah and his family are said to be descendants of Seth, and thus Seth is the progenitor of all mankind which survived the flood.
I don't believe the Bible is a text book, but it might have more information in it that you give it credit for. Atheists can say they believe in evolution, but does anyone claim to know what caused the creation of even the first single celled organisms at the beginning of evolution's chain? I don't think so.