Deuteronomy 17:14-20
When you enter the land which the Lord your God gives you, and you possess it and live in it, and you say, ‘I will set a king over me like all the nations who are around me,’ you shall surely set a king over you whom the Lord your God chooses, one from among your countrymen you shall set as king over yourselves; you may not put a foreigner over yourselves who is not your countryman.
Moreover, he shall not multiply horses for himself, nor shall he cause the people to return to Egypt to multiply horses, since the Lord has said to you, ‘You shall never again return that way.’ He shall not multiply wives for himself, or else his heart will turn away; nor shall he greatly increase silver and gold for himself.
“Now it shall come about when he sits on the throne of his kingdom, he shall write for himself a copy of this law on a scroll in the presence of the Levitical priests. It shall be with him and he shall read it all the days of his life, that he may learn to fear the Lord his God, by carefully observing all the words of this law and these statutes, that his heart may not be lifted up above his countrymen and that he may not turn aside from the commandment, to the right or the left, so that he and his sons may continue long in his kingdom in the midst of Israel.
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Jesus said that Moses allowed the Israelites to give a writ of divorce because of their hard hearts. But does that mean it was okay?
It appears to me that God was not pleased for them to have multiple wives. God may have allowed it, but that doesn't mean it was His will.
Because God blesses us, it doesn't mean that He is pleased with everything we do which is what it sounds like you're saying.
I believe that it was sin because God told them not to do it.
It must not have been looked upon as seriously as adultery because David and Bathsheba lost their child for their sin.....God was gracious because according to the law, they both deserved to be stoned for their sin. God had mercy upon them.
David paid dearly for all those wives. There was so much sexual sin including a rape within his family, murder and conspiracies.
David was a man after God's heart and he received a lot of grace and mercy, but his consequences for his sin was great.
As far as polygamy being adultery, I don't know. I'd have to think on that, my first thought though is that it isn't. But again, I've never given this much consideration in the past.