Kabbalah zu Bereschit 6:5

In the Zohar they were very stringent about this sin, and this is what it says: (Zohar with Sulam commentary, Beresheet B) 393. Of the verse, "For You are not El who has pleasure in wickedness; nor shall evil dwell with you" (Tehilim 5:5), Rabbi Yehuda said: This verse has been discussed and explained, yet come and behold: he who cleaves to and is led by the Evil Inclination is defiled and will be led further into defilement, as we have learned. 394. "The wickedness of man was great," because men committed all sorts of sins and their guilt was complete when they spilled blood in vain upon the ground. This refers to those who pollute their ways upon the earth, THAT IS, THEY SPILL THEIR SPERM IN VAIN. Thus, it is written: "Only evil (Heb. ra) all day." In another place it is written: "And Er, the son of Judah, was evil (Heb. ra) in the eyes of Hashem" (Beresheet 38:7), BECAUSE HE SPILLED HIS SEED UPON THE EARTH. THIS INDICATES THAT THE WORD RA REFERS TO THE WASTE OF SEED.