The Arizal explains that the "oneg"- pleasure that one feels from a sexual transgression, turns into "nega" - plague, by a rearrangement of the Hebrew letters.
The Arizal also teaches that every time a man misuses his seed, he is handing a sword to his urge for evil, who ascends with this sword to the man's soul root, and cuts off from there a holy soul. He then makes out of it a powerful klipah-husk, and as a result there comes a great darkness in all the heavenly worlds. The Ari adds that it is in man's power to elevate by means of his tears not only the souls that he himself brings down, but also some of the souls that other men brought down by succumbing to their physical passions. When a soul becomes liberated and ascends on high, some her fellow-souls who are enmeshed by the outside forces become envious of her, and break free from their bonds out of the power of their longing, thus ascending together with her." (Shaar Hamitzvot, perashat Noach)