In the Kitzur Shulchan Aruch (or Abridged Code of Jewish Law), it's written: "One who spills seed not only commits a major sin, but also places himself in a state of excommunication. It is considered as if he kills a person."

Why? Because he 'killed' the spiritual potentials of billions of souls, who are now trapped by the forces of evil.

Sexual impurity is a very big reason that people feel 'disconnected' from themselves and from G-d and from good, because the evil forces that are created from 'spilled seed' attach to a person at the level of daat, (which is represented by the neck) which is the 'bridge' where the physical meets the spiritual. It's like pulling the plug from the socket - disconnecting the soul from holiness.

To put it another way, these evil forces punish the people who created them by blocking G-d out at the level of the neck, cutting them off from any 'higher' spiritual concept of G-d, or emuna. These people are then only alive 'from the neck down', and live lives of unbridled physicality and materialism, cut off from any connection to their soul, or higher self.