A number of godly men in the Old Testament had more than one wife, and beyond this, some had concubines (such as Hagar in Genesis 16:1ff.), who did not have the same status as a wife. I think we must acknowledge that God did not forbid Old Testament saints to have more than one wife. Indeed, the Law even has instructions for the man with more than one wife (Deuteronomy 21:15). A king was forbidden to “multiply wives” (Deuteronomy 17:17), which appears to prohibit political alliances by means of marriages (as we see with the marriage of Ahab and Jezebel’s daughter, Athaliah, with Jehoshaphat’s son, Jehoram (2 Kings 8:18, 25-26).
In the New Testament, we find that elders must be “the husband of one wife” (1 Timothy 3:2), though there are different interpretations of what this expression means. At a minimum I would understand that an elder would not have multiple wives.
It seems to me that the Old Testament describes those marriages with multiple wives in such a way that we see the problems this creates. This is especially true of Jacob’s two wives and two concubines, and the “family” that results. It was certainly true of Elkanah’s two wives. Paul’s words in Ephesians chapter 5 make it clear that marriage was intended as a picture of the relationship of Christ to His church, and a multiple wives certainly don’t seem consistent with that!
Since the coming of Christ marriage to but one woman is clearly the ideal. Our culture has tended to reinforce this as well. It is hard to avoid the conclusion that we are more uncomfortable reading about those men who had more than one wife than God was. He did not prohibit this practice in the law, and He did not rebuke those who had more than one wife. God did rebuke Abraham for lying about his wife, and He certainly corrected Jacob on various matters, but not about his wives. Indeed, the nation Israel comes from the offspring of the four “wives” (2 wives, 2 concubines) of Jacob.
In times of old, believers had more than one wife, all at the same time. Today, even Christians have more than one wife, one at a time, by way of divorce. If there is something that God is very clear about, it is that He hates divorce (Malachi 2:16).
In conclusion, I think I would say that in Old Testament times having more than one wife was not ideal, but neither was it clearly defined or prohibited as sin. There are a number of things in the Old Testament that are difficult to understand, such as Lot offering his two virgin daughters to the men of Sodom and Gomorrah, so that they won’t rape his male guests (Genesis 19:8). I think that we will have to wait till we are in heaven to ask God about such things.