Folks who attempt to use the Old Testament to justify polygamy, should be asked to explain the 7th Commandment...
“You Shall Not Commit Adultery”
See my post 501. It's been my experience that if folks actually read the Old Testament accounts instead of relying upon hearsay, then they'd see there's little there to try to justify its practice.
While I would not agree with them, I think their answer would be that adultery is relations outside of wedlock. To their thinking, the people are married, even if it is with multiple wives, thus it would not be adultery.
Where I disagree with this, personally, is that the man had to be married to one woman first, and simply DESIRING to have a second wife (or third, fourth, whatever) would already be an act of adultery. Did not Christ say something about how the mere desire for someone other than your spouse means you already commited the sin in your heart?