First, adultery is a man having relations with a woman that is married to another man. Lev. 20:10
It is not adultery for a man to have relations with an unmarried maiden, it is fornication.
It is not adultery for a married man to take another wife if the woman is an unmarried maiden.
Jesus calls it lust for a man to look upon a married woman with unjust desire.
That is the same as the commandment of God to not covet. covet = lust = unjust desire. It is not lust for a man to desire an unmarried maiden.
Therefore, a married man may look upon an unmarried maiden as desirable to have as a wife. he may take her as a wife in addition to the wife he already has. He now has two wives. Exodus 20:10
Scripture requires that he not take away from his first wife to provide for his second wife. He is to provide for all of his wives. Exodus 21:10
If his first child is born by his hated wife and his second child is born by his loved wife, the child of the hated wife still gets the primary inheritance. Deu 21:15
Scripture even gives guidelines as to who qualifies as a polygynous wife. Lev 20:14