What makes you believe it was ok for them, just because the Bible records they participated in it doesn’t mean it’s God’s design for us.
You are correct.
Right from the beginning, Gods design was for one wife.
There were significant consequences for those in the Old Testament with multiple wives.
In Matthew 5, Jesus said even looking at, and then lusting after a woman, was already guilt of adultery in a persons heart.
Lots of evil and immorality recorded in Schripture and history, but it will always have consequences, and without repentance and forgiveness - judgement.