A frequent misinterpretation of this verse is that it universally condemns contraception, birth control, or masturbation as inherently sinful; however, the core issue was not the withdrawal method itself but Onan's wicked intent to evade his moral and legal obligation under levirate law to provide an heir for his brother Er, prioritizing personal gain over family duty and God's command, which is why the text specifies his knowledge that the seed would not be his as the motive for his displeasing act.
Genesis 38:9
King James Version (1769)
9 And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother’s wife, that he spilled it on the ground, lest that he should give seed to his brother.
Genesis 38:9 describes Onan's violation of the levirate marriage custom, where he was duty-bound to impregnate his deceased brother's widow Tamar to preserve his brother's lineage and inheritance; instead, motivated by selfishness to avoid sharing his own inheritance, he spilled his seed on the ground, an act of disobedience that incurred God's judgment, underscoring the biblical emphasis on familial responsibility, covenant loyalty, and the sacredness of continuing one's family line as part of God's providential plan.