Genesis 38:7,8 - Joseph Benson’s Commentary on the Old and New Testaments

Er was wicked in the sight of the Lord That is, in defiance of God, and his law. And the Lord slew him Cut him off by an untimely death, before he had any children by Tamar. As long life among the Jews was generally reckoned a blessing from God; so an untimely death was accounted a punishment. The next brother, Onan, was, according to the ancient usage, married to the widow, to preserve the name of his deceased brother that died childless. This custom of marrying the brother's widow was afterward made one of the laws of Moses, Deuteronomy 25:5. Onan, though he consented to marry the widow, yet, to the great abuse of his own body, and of the wife he had married, and to the dishonour of the memory of his brother that was gone, refused to raise up seed unto his brother. And this story seems to be recorded by the Holy Ghost purposely to condemn, not only his malignant and envious disposition with respect to his deceased brother, but also and especially that vile pollution of his body of which he was guilty. For, observe, The thing which he did displeased the Lord, and brought upon him the Lord's vengeance. And it is to be feared that thousands, especially of single persons, still displease the Lord in a similar way, and destroy their own bodies and souls. All such sins, at the same time that they dishonour the body, evidence the power of vile affections, and are not only condemned in the Scriptures, but by the light of nature, and were held even by the heathen moralists to be peculiarly criminal, and by the Jewish doctors to be a degree of murder. See Universal History.