To answer this, we will begin by quoting the Kitzur Shulchan Aruch.

​

"It is forbidden to discharge semen in vain. This is a graver sin than any other in the Torah.1 Those who masturbate and thus discharge semen in vain, not only do they commit a grave sin, but also one who does this is placed under a ban. Concerning such people it is said, "Your hands are full of blood,"2, and it is likened to killing a person. See what Rashi wrote in the Sidrah of Vayeishev3concerning Er and Onan, who died because of committing this sin. Occasionally, as a punishment for this sin, God forbid, one's children die when young,4 or grow up to be wicked, while the sinner is reduced to poverty."

Rabbi Shlomo Ganzfried

Kitzur Shulchan Aruch

Chapter 151:1