The Bible contains laws wrt institutions we now find offensive, such as slavery. A traditional answer given is that these laws are designed to moderate what were then prevalent customs and gradually wean people away from their practice. For example, the Bible permits the institution of slavery -- including the betrothal of one's daughter through a form of sale. Under this approach, the justification for including such an obvious moral wrong in the Bible is that it was necessary to moderate an already existing practice.
The difficulty with this approach is that the Bible contains some outright prohibitions on other behaviors. What, therefore, creates a situation in which God chooses simply to moderate behavior in the hopes that human beings will eventually give it up entirely, whereas other behaviors are banned entirely? Can we identify certain classes of behavior as creating a pattern? And -- if God disapproves of specific behaviors -- why not ban them entirely? Alternatively, how do we explain the alternative, when Rabbinic decree allows individuals to circumvent a direct Biblical ban (e.g., the creation of "prosbul" to allow debt to survive the Sabbatical Year)?
I have developed a marvelous proof. Unfortunately, I don't have time to fully outline it. This is frustrating. But hopefully I will have time to write it out more in the future.