Moses and the other prophets gave their most important knowledge hidden in the symbolism and structures of Kabbalah and Hebrew, including many instructions regarding sexuality.
I am the man who has girded his loins and will not pause til his oath is fulfilled. - Rabbi Solomon ibn Gabirol (11th century)
Rabbi Simeon sat and wept, and he said, woe if I reveal these secrets, and woe if I do not reveal them. The Companions who were there remained silent until Rabbi Abba stood and said to him, "If our master wishes to reveal these matters, is it not written "The secret of the Lord belongs to those who fear Him..." - The Zohar (presumed 13th century)
The strength of the body, the light of the eyes, and the entire life of the man is slowly being lost by too much loss of the vital fluid. - Jewish Code of Laws, Sec. Orach Chaim. Ch. 240; Parag. 14
...whoever mingles any admixture into his drop of semen by forbidden intercourse or by thinking of someone else, as if he is false to the king's seal, which is Yesod. For they are interdependent: the deed below causes damage above. - The Zohar
"No one can achieve illumination without having been previously purified. Only those that have achieved purification and sanctity can enter into the hall of illumination."