I believe that God intended man to have only one spouse (one man and one woman) and stated so in the bible. So why did the "Godly" men such as Abraham, Jacob, David and countless others have more than one wife and many concubines on top of it? When these men were tired of their sex toys they could then release them only to be replaced with new ones. Adultery is a sin, so why didn't God rebuke these men for their sins and state so for all to read?
Please see What Does the Bible Say About..Concubines?" for my previous answer about concubines. It will give you a partial answer to your question.
Although God did intend for a man and wife to become “one flesh,” he apparently tolerated multiple marriages. There is no direct scripture that condemns polygamy. In most countries today the scripture that condemns it is the one that says we are to obey the laws where we live.
Your question does show that you have a couple of misconceptions. First, when a man had several wives and concubines they could not “release them only to be replaced with new ones.” They were responsible for these women for their entire lifetime. Solomon, who had seven hundred wives and three hundred concubines, had to support these thousand women as long as he lived. Fortunately for him, he was rich and could afford it. Also, these were not necessarily sex toys. Some were political unions that may never have involved sexual relations. Clearly Abraham and Jacob had children by their concubines, but they also had fewer. Jacob kept his whole family together for life, and God had to order Abraham to send Hagar away for a specific reason.
The second misconception I noticed was your mention of adultery. Since a concubine held the legal status of a lesser wife, and since a wife was legally married to the husband, there could be no adultery. A man who is married to three women cannot commit adultery with any of them. Adultery is a sexual act with someone who is not your spouse. As long as there was a marriage between the partners before the act, then it cannot be adultery. Polygamy (marrying more than one person) is illegal in many places today. He would be in violation of that law, but not of adultery. So God could not rebuke these men for adultery because of their wives and concubines. He could, and did, rebuke David for adultery with Bathsheba while she was married to Uriah. But once he married her God even approved of the children of the union, since that became the line of kings from Solomon through Jesus.
I don’t want to give the impression that everyone should go out and marry several people because it is not specifically condemned by God. There is no indication that those who were multiply married had to give up wives when they became Christians. However, the requirement that elders be married to one wife does indicate that that was the expectation for those who did not already have more than one wife. Whether it was approved or not in the Old Testament, monogamy is expected in the New Testament.