My view is that some laws were relative and some ways to determine that is by looking at the rationale for the law and going by what the Bible itself says about it. For instance, the Bible itself tells us that the dietary laws no longer apply. Some may say that the laws on slavery no longer apply because we have no more need for slaves, voluntary or otherwise. Incest is probably another good example since it was allowed at a time when there was a limited number of mates available, but later on it was declared to be immoral. However given the nature of relative morals, I wouldn't say that the cessation of their practice shows some moral progression, because what if the need for them come up again at some future time? In the case of polygamy, what if we have a population crisis, perhaps due to war, where women outnumber men three to one?
The only biblical laws that I'm willing to call absolute are the ten commandments and those laws relating to sexual behavior.
One common problem that I encounter when getting responses to these questions is a lack of verification. When I ask those who answer that it's relative, then they usually provide some reason that is not explicitly stated in the Bible. Like for instance Catholics will tell me that polygamy is no longer moral because it was only supposed to be practice for a certain time. But where is that in the Bible? I can understand someone saying that for incest since it was presumably allowed due to the limited amount of mates, but then the Bible specifically mentions that it is no longer moral. Someone may then say of polygamy that its moral status can be inferred or that it's implicit in the New Testament but oftentimes I find those explanations to be ad hoc, with little to no biblical basis.