Musar su Genesi 49:2

If man and his wife had been able to maintain their state of innocence, the drop of semen from which a fetus develops would not be called a טפה סרוחה, an evil smelling drop. Instead, it would be called זרע קודש, holy seed. The subject of copulation and sex would not then be a subject surrounded by shame and embarrassment. It would be an act of מצוה performance no different from that of putting on phylacteries, and would be performed publicly without shame or embarrassment. An act which is not inspired, at least partially, by the evil urge is not something associated with shame or embarrassment. Now that Adam and Eve had sinned, the act became associated with shame. Jacob, who is described as the "beauty," i.e. the spiritual level of an Adam before sin, began to repair the damage caused through Adam's sin. This is the significance of the oft repeated slogan that "his bed was pure." It means that Jacob only sprouted זרע קודש, "holy seed." Jacob never experienced impurity through the emission of semen for any purpose other than procreation. He was able to say of Reuben his first born, who was conceived when Jacob was at least 84 years old, ראובן בכורי אתה כחי וראשית אוני, "Reuben you are my firstborn, my strength and the first of my vigor" (Genesis 49, 2). Jacob's principal firstborn was Joseph, however, since when he cohabited with Leah, he was under the impression that it was Rachel, the wife he worked for for seven years. This is the reason he could deprive Reuben of his rights as a firstborn and transfer that birthright to Joseph. As a result, it was no more than fair that Joseph's children would receive a double share in the land of Israel, that Menasseh and Ephraim were each considered a tribe. The spiritual identity of Jacob and Joseph is even more evident when we consider Joseph's experience with the wife of Potiphar. Joseph maintained the purity displayed by his father in all matters sexual when he resisted the seduction of Mrs. Potiphar. [He cited as the only reason that it would be a sin against G–d and against her husband. Ed.] It is a well known fact that the צדיק, the righteous individual, is the foundation, יסוד of the world. The emanation ברית העליון, is the source of the תפארת, the covenant emanating in the "highest" regions, a term familiar to students of the Kabbalah. It is a direct continuation of the forces activated in the emanation, תפארת, the emanation which mirrors the attribute of Jacob.