It's a phase transition (or: The moral way to treat Your 2nd wife, while still married to your first)

This is all just my opinion. The opinion of a low-rent armchair theologian. Take it for what it's worth.

The two major systematic theologies, dispensational and covenant, make the same category of error: they both misrepresent the continuity of biblical revelation. Dispensationalism overemphasizes discontinuity, and Covenant Theology overemphasizes continuity.

The so-called New Covenant Theology adopts the goldilocks position. It looks at the progression of special revelation (and the concomitant redemptive history) in terms of the old and the new (and better.) Kind of like, well, the Bible does. I'm a fan-boy.

If you notionally plot their respective views of history, Dispensationalism looks like a staircase, Covenant Theology is like a smooth curve, and New Covenant Theology has a vertical slope at the time of Christ (because something kinda important happened then!) in a shape that in physics we recognize as a phase transition. The New Covenant view of the NT era is not that it is like the OT era, only perhaps a bit warmer, but it is a completely different phase, like water to steam.

Covenant theology (which I greatly admire, although you might not know it) has a problem with The Law. Its overemphasis on continuity results in it making the erroneous claim that moral law of the Old Covenant is the absolute moral law for all time--in spite of much counter evidence, such as the fact that Ten Commandments are described (Ex. 34:27) as the laws of the covenant God makes with the Jews, which is not the New Covenant.

This misplaced emphasis on continuity results in Covenant Theology's position that "there really is just one convenant, for all human history, The Covenant of Grace"  a defense of which requires great exegetical-contortionist skill. It also results in a  clumsy view of the Law. The Law, they say, is conveniently broken into three disjoint groups: moral, civil, and ceremonial. This is in spite of the fact that the Bible never states that there are three types of law: There shall be not one, not two, but three, and four is right out of the question!

Since Covenant Theology acknowledges that we should should not be sacrificing animals anymore, it is faced with the harsh truth that it must relax its cherished insistence on continuity--but as little as possible. Most Covenant Theology advocates jettison the civil and ceremonial laws, keeping only the moral. One group of lunatics, however, keeps the civil law as well, and advocates for Christian control of the government (because that's always worked so well!) and nostalgic reinstatement of death by stoning, just as it was called for in the Old Testament. These are the lovable rascals known as theonomists. Pray your daughter doesn't bring one home.

It is easy to poke holes in the view that the Old Covenant moral law is the end-all of God's revelation of his moral law. Here is just one example, the Old Covenant law on how your son should treat his first wife, the one you approved of, should he get bored with her and (without divorce) marry a second:

9 If he [the father] designates her for his son, he shall deal with her according to the custom of daughters. 10 If he [the son] takes to himself another woman, he may not reduce her [wifey #1's] food, her clothing, or her conjugal rights. (Ex. 21:9-10).

This is just one example from the Law (of Moses) that described an acceptable practice and is very difficult to shoe-horn out of the category of a moral law.

Today we would find this practice incompatible with the laws on marriage given to us in the New Covenant era. We would find this practice unacceptable and immoral. (And this is one os a gazillion examples.) There is no reasonable way to argue that the moral law of God was not upgraded in the New Covenant era.

Now you could try to argue that it is only the Ten Commandments that are absolute. Leaving aside that even this is hard to defend, the new problem is that you really are now talking of four types of laws:

1. Ceremonial (toss 'em)
2. Civil (toss 'em if you are not a lunatic theonomist)
3. The Ten Commandments (keep 'em)
4. Moral Laws that that were considered consistent with the Ten Commandments but are now considered immoral. Because reasons. (toss 'em)

A house of cards.