Genesis 29:28

He gave him Rachel... to wife also. — After the monogamy of Abraham, and the stricter monogamy of Isaac, how came Jacob to marry two wives? Abravanel says that as Esau ought to have married Leah, and Jacob Rachel, he acted only as his brother’s substitute in taking the elder, and was still free to marry the younger sister, who was his by custom, He thinks also that Jacob, recalling the promise of a. seed numerous as the dust (Genesis 28:14), and seeing how near the family had been to total extinction in the days of his father and grandfather, desired to place it on a more secure basis. More probably, even after Leah had been forced upon him, Jacob regarded Rachel as his own, and as polygamy was not actually forbidden, considered that he was only acting justly by her and himself in marrying her. He had seen Esau blamed, not for marrying two wives, but for taking Hittites; and his love for Rachel would make him need but little argument. The only other alternative, namely, to have divorced Leah, would have been worse, and happily divorce was not a practice as yet introduced.