No such "have to"...
God saw all that He had made, and behold, it was very good. It WAS very good, pre-fall. Man had free will, but he WAS still good, with the potential to choose sin. This potential is not a flaw. If it wasn't there, we'd be automotons.
The alternative? Just toss the basis of all Christian religion, and that is the whole goal anyway, isn't it?
If you assume that the Christian idea of sin and forgiveness is inextricably dependent on the existence of a literal Garden of Eden, a literal tree of the knowledge of good and evil, a literal talking snake, a literal tree of life, a literal inability of God to locate his creation in the Garden, etc., etc., then I suppose you would be "tossing" the basis for Christianity.
But of course, for a great many Christian theologians throughout history, such literalism has not been the sine qua non of Christianity. This is undoubtedly because, as allegory, the Genesis account is a more compelling and inspired rendition of the nature of man and his relationship to God than it would be if taken as a mere event catalog.