Onan

…it as the sin of Onan, who in the Old Testament was censured for spilling his seed; and the Roman Catholic church still officially condemns masturbation as a mortal sin.

…the Old Testament story of Onan (Genesis 38:8–10), Judah ordered his son Onan to sleep with Onan’s recently widowed sister-in-law, but Onan refused on the ground that “the descendants would not be his own, so whenever he had relations with his brother’s wife, he let [the seed] be lost on…