Good question.
The word "polygamy" isn't actually quite right, and that needs to be explained. What "polygamy" means is any sort of multiple-person relationship of three or more, which includes a lot of relationships which are clearly sin according to the Bible and the Bible's clear definitions.
Biblical marriage is one man and one woman. Marriage can only be between one man and one woman. The Bible shows us many families in which the man has multiple marriages, or multiple covenants with separate women. So, if a man has two wives, he has two marriages with separate covenants.
This is the best Biblical definition of what is called "polygyny," rather than "polygamy." So polygamy is a larger umbrella term, but it is a worldly, non-Biblical term. "Polygyny" is the word we use, which is a much narrower term which can only include one man.
It's a huge topic and once someone begins to understand the reality of the difference between the way marriages and relationships in the Bible are described - versus how we do things in the secular western world - the conversation just gets deeper. It's a huge subject and for me it has brought a lot of peace when realizing that the modern western church has been accidentally misinforming us all for so long.