Please give examples.
Just google “Rabbi Gershom ben Judah” for the explaination. His writing are the most clear on the subject.
In EXTREME brief, there are numerous statements in the Torah that polygamy was a bad idea, available in special circumstances only, and numerous examples of the trouble it caused in general, from murder to inter-sibling jealousy, rape, and sibling murder.
Contrary to popular belief, polygamy is never “sanctioned,” merely tolerated (under specific circumstances), even under strict Karaite reading of the Torah.