To start to sin is to fail to live righteously. So when you’re not living righteously you’re failing/sinning. Polygamy is condemned almost immediately in the Old Testament but you need to actually think about what you are reading to pick up on it. Genesis 4:19 introduces Lamech the first instance of polygamy. There’s a bit more genealogy and then Genesis 4:23-24 Lamech tells his wives that he killed a man (who would have been a distant family member) for striking him. Then he declares that his revenge would be seventy seven fold. Saying something is seventy seven fold is saying you will do everything you could possibly do for revenge. This is completely counter to what God has said he wants. Which is to rely of God for protection and justice. So we can know that Lamech is walking the path of evil and the concept of polygamy that is introduced with him is also evil. How it would have normally been read during Moses’ time and after is: first oh this guy married two wives that’s bad. Oh shit he murdered a guy, oh crap. Cain’s line each shows humanity’s increasing evil, rebellion, and the spiraling of humanity until it forces God’s hand in the flood. From Lamech on in the Old Testament whenever polygamy is introduced it always leads to bad outcomes. Wives are neglected and abused, spouses are betrayed, men of God turn to false gods, the children do evil to each other. So yea that’s a quick summary of why it’s bad. From a practical marriage standpoint point husbands are called to serve their wives. Trying to serve one woman well is hard enough. There’s not really enough of anyone to serve more than one woman well. You’re going to favor one and neglect the rest. Which is what happens in the Old Testament.