One beloved, and another hated - That is, one loved less than the other. This is the true notion of the word hate in Scripture. So Jacob Hated Leah, that is, he loved her less than he did Rachel; and Jacob have I loved, but Esau have I Hated, that is, I have shown a more particular affection to the posterity of Jacob than I have to the posterity of Esau. See the note on Genesis 29:31. From this verse we see that polygamy did exist under the Mosaic laws, and that it was put under certain regulations; but it was not enjoined, Moses merely suffered it, because of the hardness of their hearts, as our Lord justly remarks Matthew 19:8.
Moses did not originate the rights of primogeniture (compare Genesis 25:31), but recognized them, since he found them pre-existing in the general social system of the East. Paternal authority could set aside these rights on just grounds Genesis 27:33, but it is forbidden here to do so from mere partiality.