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.
Hi again AB! Your questions are covered in detail in the Catholic Catechism. There are several stages to the moral law. Here is a summary from the Church teachings:
1962 The Old Law is the first stage of revealed Law. Its moral prescriptions are summed up in the Ten Commandments. The precepts of the Decalogue lay the foundations for the vocation of man fashioned in the image of God; they prohibit what is contrary to the love of God and neighbor and prescribe what is essential to it. The Decalogue is a light offered to the conscience of every man to make God's call and ways known to him and to protect him against evil:
1963 According to Christian tradition, the Law is holy, spiritual, and good,14 yet still imperfect. Like a tutor15 it shows what must be done, but does not of itself give the strength, the grace of the Spirit, to fulfill it. Because of sin, which it cannot remove, it remains a law of bondage. According to St. Paul, its special function is to denounce and disclose sin, which constitutes a "law of concupiscence" in the human heart.16 However, the Law remains the first stage on the way to the kingdom. It prepares and disposes the chosen people and each Christian for conversion and faith in the Savior God. It provides a teaching which endures for ever, like the Word of God.
1964 The Old Law is a preparation for the Gospel. "The Law is a pedagogy and a prophecy of things to come."17 It prophesies and presages the work of liberation from sin which will be fulfilled in Christ: it provides the New Testament with images, "types," and symbols for expressing the life according to the Spirit. Finally, the Law is completed by the teaching of the sapiential books and the prophets which set its course toward the New Covenant and the Kingdom of heaven.
1965 The New Law or the Law of the Gospel is the perfection here on earth of the divine law, natural and revealed.
1968 The Law of the Gospel fulfills the commandments of the Law. The Lord's Sermon on the Mount, far from abolishing or devaluing the moral prescriptions of the Old Law, releases their hidden potential and has new demands arise from them: it reveals their entire divine and human truth. It does not add new external precepts, but proceeds to reform the heart, the root of human acts, where man chooses between the pure and the impure,22 where faith, hope, and charity are formed and with them the other virtues. The Gospel thus brings the Law to its fullness through imitation of the perfection of the heavenly Father, through forgiveness of enemies and prayer for persecutors, in emulation of the divine generosity.23
1972 The New Law is called a law of love because it makes us act out of the love infused by the Holy Spirit, rather than from fear; a law of grace, because it confers the strength of grace to act, by means of faith and the sacraments; a law of freedom, because it sets us free from the ritual and juridical observances of the Old Law, inclines us to act spontaneously by the prompting of charity and, finally, lets us pass from the condition of a servant who "does not know what his master is doing" to that of a friend of Christ - "For all that I have heard from my Father I have made known to you" - or even to the status of son and heir.31