Who among Jacob's sons was the most unchaste and promiscuous?
Reuben, Jacob's eldest son, was the most unchaste and promiscuous. He committed adultery with one of Jacob's wives. Because of this deed, Jacob later declared that Reuben would not receive the birthright or blessing and would not be esteemed as the eldest.
Les explains that when Jacob gives blessings to his twelve sons, the one son who was the most unchaste, the most promiscuous, and did the most awful deed was Reuben, the eldest. Reuben committed adultery with one of Jacob's wives. Evidently nothing was ever said about it until Jacob's deathbed. At that time Jacob brought it to mind and declared that Reuben would not have the birthright or the blessing and would not be esteemed as the eldest because he committed adultery with one of his wives. Les notes that seemingly no one else ever knew it.