Just because God allows or permits his children and image-bearers to stray from his original design for them, does not mean he sanctions their straying or places his imprimatur on it.
For example, God's design for the nation of Israel was for her to be a theocracy, not a kingdom in the usual, worldly sense. YHWH wanted to be her king, and he provided Israel with prophets, elders, military leaders, judges, and--of course--priests, to provide her with all she needed to accomplish his will and to be a light to the Gentiles surrounding her.
Samuel summoned the people of Israel to the Lord at Mizpah and said to them, “This is what the Lord, the God of Israel, says: ‘I brought Israel up out of Egypt, and I delivered you from the power of Egypt and all the kingdoms that oppressed you.’ But you have now rejected your God, who saves you out of all your disasters and calamities. And you have said, ‘No, appoint a king over us.’ So now present yourselves before the Lord by your tribes and clans" (1 Samuel 10:17-19 NIV).
I'm sure we would have little difficulty in citing other ways in which God accommodated himself to Israel and her leaders, not because his design for her had changed but because sometimes he gives his children enough rope with which to hang themselves. God's decretive will cannot be altered, and his revealed will is not a suggestion. When his image-bearers considered his permissive will to be one-in-the-same with his revealed will, the resulting "wiggle room" originated in them, not God.
A common aphorism is "Be careful what you wish for," and its implication is clear: We are allowed by God to flaunt his design, but we do so at our own peril. Actions have consequences, and when we confuse God's permissive will with his revealed will, we often--if not always--suffer the consequences.
Yes, God gave to King David the house that belonged formerly to his predecessor, Saul, which included Saul's wives, but again, God's doing so was an accommodation to David as a king among similar kings in the surrounding nations. No self-respecting king back then would be without his concubines, and yet God's revealed will to Israel, including Israel's leaders, was "one husband and one wife for life."
Some of the patriarchs of Israel (and perhaps many of Israel's kings) flaunted God's revealed will in this area because they confused his latitude with his permission. We have only to look at King Solomon's reign to realize his fascination with multiple lives was largely his undoing, spiritually:
As Solomon grew old, his wives turned his heart after other gods, and his heart was not fully devoted to the Lord his God, as the heart of David his father had been. He followed Ashtoreth the goddess of the Sidonians, and Molek the detestable god of the Ammonites. So Solomon did evil in the eyes of the Lord; he did not follow the Lord completely, as David his father had done (1 Kings 11:4-6 NIV, my bolding).
Clearly, God sometimes overlooks (or "winks at") his creatures' ignorance (see Acts 17 in the KJV at Acts 17:30 and passim), but he nevertheless does two things. First, he allows their sins to find them out (see Numbers 32:23 and passim). In other words, they reap what they sow. Second, he allows us to feel the brunt of our misguided (and often sinful) actions with unpleasant consequences.
Turning to God's Word in the New Covenant, we can invoke no greater authority about God's purpose for marriage than to quote Jesus, who said,
“Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female,’[see Genesis 1:27] and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’[see Genesis 2:24]? So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate.”
Also in Mark 10, Jesus reaffirms God's design for marriage from the very beginning: One husband and one wife for life. Two become one flesh. Not three or four or five become one flesh, but just two and only two. Remember, too, that the context of both Matthew 19 and Mark 5 involves the subject of divorce. In both places, Jesus reaffirms the sacredness of the bond between husband and wife.
Notice as well that Moses, not God, allowed for divorce, and he did so because of the hardness of heart that plagued the Israelites. Interestingly, the only "out" that Jesus allowed for divorce was marital infidelity and the breaking of the Sixth Commandment: You shall not commit adultery.
In short, I challenge anyone who thinks that God was somehow "for" polygyny (which is the correct term for multiple wives) to examine the life of every believing (and unbelieving!) man who flaunted God's design for marriage. In taking multiple wives he was inviting heartache, disappointment, disillusionment, conflict, and spiritual declension to become part of his family's life.
That God permits multiple marriages is not the same as God sanctioning multiple marriages.
In conclusion, as an author whose name I cannot recall at the moment observed, the pattern the apostle Paul speaks of in Ephesians 5 did not originate in marriage but in the relationship between Christ and the church universal. Too often we use the former as the pattern for the latter, which is a mistake. As Paul said,
This is a profound mystery—but I am talking about Christ and the church. However, each one of you also must love his wife as he loves himself, and the wife must respect her husband.
God's love for his saints originated in his character and in his counsels in eternity past. The sacrificial love that God would one day demonstrate at Calvary, before a watching world, preceded the love of the first man Adam for his wife Eve. The singularity of that bond between Christ and his church is, I believe, more than enough proof that multiple marriages were not part of God's good, acceptable, and perfect will for his children in any age. If in rare instances God seems to have placed his imprimatur on polygyny, you can rest assured it is because he cared more about the woman's well being than the man's pleasure, as evidenced, for example, in the levirate (see Deuteronomy 25:5-10).