For starters, the story is pretty obviously allegorical because of the way it is constructed, but if you want to look at the literal interpretation, there are a couple of issues to consider.
Adam and Eve were not the first man and first woman, read Genesis 1, near the end of that chapter, god creates man and woman – at the same time – in his own image and likeness, out of nothing, then tells them to make lots of babies. These humans are never identified. Early in chapter 2, god creates Adam out of the dust of the earth and breathes life into him, then creates Eve through surgery. There is no indication that the first two humans were Adam and Eve, they were created in a different manner, so Adam and Eve were the second couple created. That is how the text reads.
More importantly, eating the fruit of the tree if knowledge of good and evil was not a "sin". God told them the fruit would kill them, so he explained why they should not eat it, not "because .i say so". Before they ate it, they did not understand right and wrong, the gained the ability to discern that from eating the fruit
. In order to "sin", you have to understand morality, but in this case, we have the circular argument that gaining an understanding of morality becomes a sin. In the US Constitution, they specifically prohibit this kind of ex post facto
conviction. You cannot do a morally wrong thing ("sin") if you do not know that it is wrong, "sin" must be freely chosen, not accidental or from duplicity.
Genesis was compiled by Rabbis, it is a Jewish text, the Jews do not have a concept of "original sin", that was grafted on by the christians. The Eden story symbolizes the "point" where humans stopped being animals that functioned primarily on instinct and started to use reason to deal with moral issues. There is nothing in there about being irrational or flawed (in fact, gaining moral understanding is exactly the opposite of irrational).