Where in the Bible does it say that Abraham and Israel were committing sin by their polygamous marriages? It doesn’t. Maybe your preacher’s interpretation of the Bible says it is sinful, but it isn’t in the Bible.
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.
It doesn't. However, like divorce which was legal under the Mosaic Law, Jesus raised the standard; forbidding divorce and affirming the proper marriage of one man & one woman.
Leviticus 18:18(ESV)—And you shall not take a woman as a rival wife to her sister, uncovering her nakedness while her sister is still alive.
Deuteronomy 17:17(ESV)—And he shall not acquire for himself many wives, lest his heart turn away, nor shall he acquire for himself excessive silver or gold.