If a man has two wives, and he loves one but not the other, and both bear him sons but the firstborn is the son of the wife he does not love, when he wills his property to his sons, he must not give the rights of the firstborn to the son of the wife he loves in preference to his actual firstborn, the son of the wife he does not love. Deuteronomy 21:15-16
Polygamy was practiced so God made laws to protect those wives (and their children) who were not the husband’s favorite. The firstborn son was the firstborn son – no matter who the mother was. This was God’s way of giving honor to those women who were not given such love from their husbands. It is strange, but I do love how God always works to protect those who struggle in this world. He is working to protect you, and maybe to give honor and dignity to someone else through you. That is the way we grow stronger as a community.