“If a man has two wives, the one loved and the other unloved, and both the loved and the unloved have borne him children, and if the firstborn son belongs to the unloved,
15. If a man have two wives. Inasmuch as it is here provided that a father should not unjustly transfer what belongs to one son to another, it is a part and supplement of the Eighth Commandment, the substance of which is, that every one’s rights should be preserved to him.
This law restrains men from disinheriting their eldest sons out of mere caprice, and without just provocation. I. The case here put (Deu 21:15) is very instructive. 1. It shows the great mischief of having more wives than one, which the law of Moses did not restrain, probably in hopes that men's own experience of the great inconvenience of it in families would at last...
Commenting on Deuteronomy 21:15-17
If a man have two wives,.... Which is supposed, but not approved of, though permitted because of the hardness of men's hearts; for it was not so from the beginning, when only one man and one woman were created, and joined together in marriage; but as it was connived at, and become customary, a law is made to prevent confusion, and preserve order in families...