Marriage is the union of male and female.
[Mat 19:4, 6 ESV] 4 He answered, "Have you not read that he who created them from the beginning made them male and female, ... 6 So they are no longer two but one flesh. What therefore God has joined together, let not man separate."
Now you will find many polygamous families, but there are no polygamous marriages in the Bible.
For example, Jacob married Leah and then seven days latter he marries Rachael. That is to say, Leah is not married to Rachael, and Rachael is not married to Leah. The Bible doesn't make a distinction between these marriages. Jacob, after that week, had two wives, but each of those wives only had one husband.
Christians like to pretend that there is something, fundamentally different going on, but the Bible doesn't make that distinction.