I forgot where I read / watch this, but this is not my original idea. The idea is that Christians have to believe in the creation and fall account in Genesis as literal history because otherwise, the concept of "sin" is moot and Jesus wouldn't need to come and save the world from sin. And if they think that Genesis is literal truth, then there are plenty of questions that they need to answer such as:

1. Without star, how could there be "light"?
2. What is "firmament"?
3. How could plants and trees be created before the sun (which is a literal star)?
4. Which of the two contradicting creation stories is accurate?
5. If the tree of life gives eternal life and the tree of knowledge introduce death, what would happen if Adam and Eve didn't eat any of them? Will they live eternally or die?
6. In the creation story, isn't the snake telling the truth? They didn't die right away and they become like God (knows about good and evil).

Those are just off the top of my head and just from the creation episode alone. If they think that it's metaphorical, then what's the point of Jesus? All Christians want to debate are the finer detail of their myths like whether Jesus was resurrected, whether the accounts in the gospels are true, etc. But when you look at it, the entire religion is like an inverted pyramid standing on a lone brick which is the creation myth. Once it shatters, the entire religion should come down with it.