Like the spilling of blood
Excerpts from an article at www.jewishsexuality.com
The Hebrew word for semen is zera, which means seed. This seed contains not only the physical and spiritual blueprints of life, but also the life force itself. Each discharge of semen contains hundreds of thousands of souls. Each microscopic drop is more than a potential life. It is already a living soul. For this reason, the sin of spilling semen in vain is considered like the spilling of blood – like taking the life of a person. Not just the life of any person, but the life of the child [somewhat] of the man who commits the sin. (Niddah 13A; Even HaEzer, 23:2)
Kabbalistic sources explain that that wasting of semen gives strength to the forces of impurity and evil in the world. This mystical concept is explained in depth in the book, "Secret of the Brit." Here, we will only mention that the impurity that a person adds to the world through sexual transgression, not only causes many forms of personal suffering, but also creates a barrier between the man and G-d, dulling his spiritual sensitivities, and turning his prayer and Torah learning into heavy, half-hearted endeavors which bring no satisfaction nor joy.