Why weren’t concubines considered sinful in the Bible? Vs. why is it sinful now to have more than one wife/lust over other women/men?
While many biblical characters have concubines and marry multiple women I have never found a place where God says “this is good”. Instead God gives the Israelites laws which allow for this practice as a means of curtailing its potential abuse.
See just because God allows for something does not mean that God approves of something. We have similar legal structures. We allow for a certain amount of drunkenness legally. Does that mean that the lawmakers approve of drunkenness? No. But our government understands that people are going to get drunk. So the laws are there to try to keep people from getting TOO drunk.
Similarly God’s laws over multiple marriages, concubines, and slaves are there. But they are there not because God approves of those behaviors. But because God understands that we are sinful and that if not given laws to limit our sins we would run amok.
Why it is sinful goes back to the very nature of marriage. Ephesians 5:32 clues us in to the fact that marriage is a symbol of Christ and the church. Therefore Adam and Eve are meant to show the singular unity of God. One God, one church, united together by covenantal love. That is why polygamy, polyamorous relationships, or bigomy do not glorify God.