Abraham didn’t HAVE a polygamous marriage. He had a concubine, because of Sarah’s lack of faith. That situation ended in disaster that is still having repercussions today.
Isaac was not polygamous.
Abraham’s wives had different legal status, that is true, but he was still polygamous.
And, you can’t deny that Jacob/Israel was polygamous. So was Moses. We know of at least two wives, Zipporah and the Ethiopian woman.