At first glance, it sounds reasonable enough that even if Jesus was the Son of God and not merely a good human teacher, He might have chosen to accomodate the ignorance of his listeners, speaking in mythical terms that they would understand. After all, He told parables, didn't He?
There's a problem with this idea, however. If you look at the gospels, when Jesus used parables and metaphors, it's generally indicated in or obvious from the context that He is using figurative language. Phrases such as "I am the Door", for instance, would not have been understood literally by any of His hearers, and as He goes on to expound on the meaning of His statement it's clear that He's using a metaphor. Elsewhere it's even more obvious, with phrases like, "And He spoke to them in parables, saying..." and in some cases, as in the case of the parable of the sower, a full interpretation of the parable follows, with each symbolic element assigned its literal meaning.
There are no such indicators, however, when Jesus speaks to the Pharisees about divorce and invokes the example of Adam and Eve. He's not merely using Adam and Eve as metaphors, here: He's arguing that because God literally created a literal man and a literal woman and gave them to each other in a literal first marriage, therefore once a man and woman have been joined in marriage, God means them to belong exclusively to one another and to take their relationship seriously. And in the process of making this argument, Jesus quotes directly from Genesis 1:27 and 2:24, treating both chapters as historical and authoritative accounts of what God actually said and did at the beginning of creation.
If in this passage Jesus were merely making reference to Genesis as a shared myth with no basis in literal historical fact, then wouldn't actually be proving anything, any more than I would be proving something if I said, "The story of Snow White proves that eating apples is dangerous, and therefore you ought not to eat apples." Of course, one might say, the Pharisees were part of a scientifically ignorant culture and didn't know that the story of Adam and Eve wasn't literally true, so it was okay for Jesus to use the myth in this way to convince them. But that puts Jesus in the position of exploiting the Pharisees' scientific ignorance and using a false argument to persuade them. And it also makes His words irrelevant to us today, because if God did not actually create Adam and Eve as distinct male and female human beings whose union generated the rest of the human race, then the whole basis for Jesus's "one man/one woman/marriage is sacred" argument collapses.
But there's an even more serious problem, involving not just this one passage but indeed the whole message of the Bible from beginning to end. What the Bible tells us, beginning with Genesis and carrying straight through to the end of Revelation, is that sin is a real and terrible problem in the heart of humankind, and that since the events of Genesis 3 all creation has been "groaning" on account of it. In other words, the Bible insists that at some definite, historical point in the past, mankind made a conscious, willful decision to rebel against God and His commandment, thus bringing about the moral corruption of humanity, the inevitability of physical death, and all the disastrous faults we see in the world around us. This is why humanity needed sacrifices for sin (a practice we see beginning with Cain and Abel) and ultimately, a divine, infinite, perfect Redeemer to pay sin's penalty and offer us salvation.
However, if there was no Eden, no Adam and Eve, and no literal space-time fall, there is no such thing as sin -- only faulty creation on God's part. We are left to conclude that somehow God neglected to build into the process of evolution a safeguard to keep it from going wrong, leaving man and nature mere victims of a faulty evolutionary process, and a careless or short-sighted or indifferent God the real author of the phenomenon we call sin. In which case the whole idea of God sending a Redeemer in the form of His Son Jesus Christ is sheer mockery -- God trying to cover up His own mistakes and pin the blame on us.
But the Bible tells us very firmly that God did not make creation as we see it today: rather, He created it "good" and it was only corrupted after humanity fell. Both the Old and New Testaments repeatedly assert that God is not the author of sin; rather, He is completely opposed to and set apart from sin, and it is antithetical to His very nature. At the same time, the Bible insists, we human beings are far from being victims of chance or evolution -- rather, we are all directly and materially responsible for our own moral and spiritual choices, and answerable to a holy God.
Neither the divine righteousness nor the human moral responsibility described in the Bible are possible, however, unless there was at some point a literal commandment that a literal Adam and Eve literally chose to violate, and therefore a literal space-time fall in a literal Eden. Otherwise, the whole concept of "sin" is meaningless. We can't talk about morality, only about the way things have evolved to be -- and as such we have no basis on which to judge any creature's behavior as good or evil, right or wrong, because all of us are nothing more than soft machines programmed by our biology, with no reason to resist any of our biological impulses.
My personal conviction is that the story of Adam and Eve as the father and mother of all human beings, and the Fall as the origin of sin and as the reason for man's estrangement from God, may well be powerful in a mythic or symbolic sense, but in the Biblical context it has no meaning unless it actually happened. Without that element of factuality, the whole Biblical narrative of sin and redemption falls apart. Indeed, without that historical narrative and its climax in the literal death and literal resurrection of Jesus Christ the Son of God, the Bible itself would tell us that there is no hope, no meaning, and no value in the Christian faith for anyone, and that Christians are to be "pitied above all men".
NOTE: For anyone interested in a more cogent and thorough examination of these ideas, my brother Stephen L. Anderson has written an article entitled "Can Myth Save the Miraculous?" to be published in a forthcoming issue of Philosophy Now. Recommended.