Understanding the fall was of Adam was caused by race-mixing, is the key to understanding Genesis. Genesis is heavily written in parables but by reading the rest of the Bible, you can piece together what truly happened. The Serpent is an allegory for the Fallen Angels and a Fallen Angel was there in the Garden with Adam and Eve. This Fallen Angel seduced Eve and the result was Cain. Adam accepted Eve back and that was his sin, which caused his fall and the fall of all adamkind. The rest of the Bible is his descendants, through Seth against the race of Cain.