To answer the question "Why must the Bible be taken literally?"
I think a better question is "how can a person be a Christian and not take the Bible literally?"
If you believe that God exists, and your evidence for that is the Bible, how can you accept that part as being true, but not accept other parts. Either the bible is correct, and God does exist, or it is false and the God described in the Bible does not exist.
If you agree with the teachings of Jesus, then surely you have to agree with all the teachings, or are you happy to admit that Jesus was wrong about some things? For example Jesus says 'Do not think that I have come to abolish the Law of Moses or the Prophets. I have not come to abolish them, but to fulfill them.' - Matthew 5:17
Jesus is saying that what is written in the old testament is true. If he is wrong about that, how do you know he's not wrong about other things? How can you believe only the teachings you agree with, and if you do, are you following Jesus, or are you following yourself?
If Jesus is right, then do you agree that it is a sin to pick up sticks on a Sunday, or is it more likely that the whole book is mythology and should not be used as a moral guide, or as truth.