My initial response to someone looking to justify polygamy from the lives of a few Patriarchs in the Old Testament is to point them to the culture from which Abram came out of. In Mesopotamia, polygamy was normal if a man’s first wife was barren. The first wife was even involved in selecting the second wife. Those cultural norms reveal a good reason for what they did. Also, since there was little to no special revelation (written books) from God at this time in history it’s no surprise that the patriarchs continued in their old ways. They likely would’ve had the Genesis account of God instituting marriage with one man and one woman. From the Bible’s perspective, Adam never married another woman. This is a worthy record enough for all to follow after instead of cultural norms. Their polygamous ways were merely residue from their previous life. They had not yet fully left their former life as God commanded.
On the practical side, we see nothing but turmoil, strife, and contempt when it comes to polygamy. Hagar ended up being despised by Sarai. Ishmael and his descendants throughout history have been a scourge to themselves and the rest of mankind, just as God said. Sister rivalry took on new heights between Leah and Rachel. Even leading them to hand over their maids to Jacob as the competition for bearing children raged on.
Most importantly of all, God never condones it. God’s Word never suggests it. And as Jesus interprets the command against adultery, He points out that any lustful intent in your heart towards a woman that is not your wife is adultery. This commandment is impossible to keep in polygamy. It is nonsensical to think God would condone a lifestyle that necessitates breaking His commands over and over again. Even one of the qualifications of being an elder in the church laid out in 1st Timothy by the Apostle Paul is that he must be a one woman man.
Soli Deo Gloria