The primary function of the scriptures is to make Christ known. Marriage is a symbol of Christ and the church. That is it's primary function.
Eph 5:32 This is a great mystery: but I speak concerning Christ and
There is only one true man. Christ. He has taken Israel as a bride and the church as a bride. The two brides represent one bride with a dual nature. She is the prostitute (having gone after false gods) who has been made to be a virgin (righteous before God) by the works and blood of the man.
There is no moral aspect to being married or remaining single or Paul could not have recommended both. There is no moral aspect to divorce, or God would not tolerate it. Marriage is a symbol of Christ and the church that men could not fully participate in because of the hardness of their hearts, and so God permits divorce. But there is no better symbol of Christ and the church than a faithful man with his bride.
(This sin involved with divorce is the sin of not forgiving and being reconciled, which many are guilty of without being married. God hates this like divorce such that he doesn't want your sacrifices until you are reconciled.)
Since the man represents Christ, all may "marry him". But since the woman represents us, for us there can be only one God. The law perfectly illustrates this.
And the law has a shadow of the "good things coming". Heb 10.1
Using David as a type of Christ, he was an adulterer. Christ was figuratively an adulterer since he took the bride of the god of this world, and put her husband to death. The first born Child of the union represents Christ in the flesh who dies like David's firstborn of Bathsheba. His second son represents Christ as the second Adam, obtaining the promise that the first lost.
David also dies, not being able to build the temple, because there was blood on his hands. Here he represents Jesus in the flesh who puts us to death by his righteousness:
Joh 3:19 And this is the condemnation, that light is come into the
world, and men loved darkness rather than light, because their deeds
Christ, as the adulterer, is put to death, by stones (the word of God). But Solomon, representing the resurrected Christ, is fruitful and multiplying.
Why is an unmarried woman sleeping with a married man not adultery? Because it is marriage. They have become one flesh.
1Co 6:16 What? know ye not that he which is joined to an harlot is one
body? for two, saith he, shall be one flesh.
In sensus plenior there are four layers of meaning. This means that there are four legitimate interpretations each having a different scope. The voice of the priest speaks of the eternal Son, the prophet of Christ in the flesh. The voice of the prophet speaks God's judgement upon the literal historical. The voice of the king is the literal-historical.
When speaking of the Son, all the men are one man and all the women are one woman. In this scope, there is only one woman for one man. When speaking of Christ, the Father has chosen a bride who is a prostitute (Israel and the gentiles) and makes her a virgin. In this scope, we see a polygamous relationsship. Many are called (wooed). But even Jesus's earthly life is but a shadow of the reality of the Son where there is only one woman.
So why does God tolerate polygamy (even today) yet Jesus seems to enforce monogamy. Apparent contradictions are riddles.
In the voice of the judge:
The sin of Eve (which did not bring sin into the world as Adam's sin) was done because she was deceived by the serpent. She was led to believe that because animals live instinctively, eating, procreating, etc by instinct; that she could also live instinctively like an animal. But she was created in God's image and likeness, capable of responding to His word.
We have clothing, grace before meals, marriage, the Sabbath, etc. to remind us that we are more than animals. We do not go naked, eat instinctively, procreate instinctively, nor pass the time instinctively. We are born as animals, and must be taught God's laws and character, and even be born again before we can even "see the kingdom of God" to live according to his law in his grace. (Obeying God is NOT a work! It is acknowledging Him as God Ro 1:18ff).
Polygamy, like divorce, is tolerated because many men still ARE animals, lacking the spirit of God which is given with a new birth. The law tolerated polygamy, but not for kings  who were enlightened by God, only for those who still were living instinctively like animals.
Jesus reinforced the teaching that we are more than animals by reminding us that in the beginning there was one man and one woman. And later Paul calls us to the higher living saying that leaders of the church should only have one wife, where a man with many wives obviously does not understand the call to higher living.
The purpose of marriage is to be a "dinner theater" as a witness of Christ and the church. The polygamist, like the adulterer, lives for the flesh.
What does this do to the idea of a Holy Marriage?
It puts it in a proper perspective. Holy means separate. The Holy marriage is one which has been separated from various marriages that the world offers. It is one which is dedicated to reflect Christ and the Church, not only in the ceremony, but in the man's love for the woman and her honoring her husband.
Though becoming one flesh make A marriage, becoming one flesh in Christ makes a Holy marriage.
Deut 17: 15 Thou shalt in any wise set him king over thee, whom the
LORD thy God shall choose: one from among thy brethren shalt thou set
king over thee: thou mayest not set a stranger over thee, which is not
thy brother. 16 But he shall not multiply horses to himself, nor
cause the people to return to Egypt, to the end that he should
multiply horses: forasmuch as the LORD hath said unto you, Ye shall
henceforth return no more that way. 17 Neither shall he multiply
wives to himself, that his heart turn not away: neither shall he
greatly multiply to himself silver and gold.