Genesis 38:9

Gen 38:9 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.

Ver. 9. When he went in unto his brother’s wife.] God, for the respect he bears to his own institution of marriage, is pleased to bear with, cover, and not impute many frailties, follies, vanities, wickednesses that are found between man and wife. Howbeit, there is required of such a holy care and conscience, to preserve between themselves, by a conjugal chastity, the marriage bed undefiled; taking heed of an intemperate or intempestive use of it: which by divines, {a} both ancient and modern, is deemed no better than plain adultery before God. Qui cum uxore sua, quasi eum aliena, concumbit, adulter est, saith that heathen. {b} Onan’s sin here was self-pollution, aggravated much by his envy that moved him to it, expressed in these words, "lest he should give seed to his deceased brother." And the more sinful was this sin of his in spilling his seed; because it should have served for the propagation of the Messiah; therefore the Lord slew him: as also, because he was not warned by his brother’s punishment. {c}

{a} Intemperans in coniugio, uxoris suae adulter est. - Aug. In uxorem alienam omnis amor turpis est, in suam vero nimius. - Hieron.

{b} Seneca.

{c} Hebraei inquiunt perinde ut homicidam, reum esse qui temere semen profundit. - Mercer., in loc.

John Trapp, A Commentary or Exposition upon All the Books of the Old and New Testament (1647–1656) — public domain. Text from a community e-Sword module.

38:9-10 Onan married Tamar, but was not willing to provide an heir for his brother. He would have sex with Tamar but not fulfill his responsibility to his dead brother (38:8).

Tyndale Open Study Notes — © Tyndale Open Resources, licensed CC BY-SA 4.0. Text via the HelloAO Bible API.

Verse 9. Onan knew that the seed should not be his] That is, that the child begotten of his brother's widow should be reckoned as the child of his deceased brother, and his name, though the real father of it, should not appear in the genealogical tables.

Adam Clarke's Commentary on the Bible (1810–1826) — public domain. Text from a community e-Sword/MySword module.

And the thing which he did displeased the Lord,.... Being done out of envy to his brother, and through want of affection to the memory of his name; and it may be out of covetousness to get his estate into his own hands, and especially as it frustrated the end of such an usage of marrying a brother's wife; which appears to be according to the will of God, since it afterwards became a known law of his; and it was the more displeasing, as it was not only a check upon the multiplication of Abraham's seed as promised, but since the Messiah was to come from Judah. This was doing all to hinder it that lay in his power:

wherefore he slew him also; in like manner as he had slain his brother, Gen 38:7.

John Gill's Exposition of the Whole Bible (1746–1763) — public domain (CC PDM 1.0). Text via the HelloAO Bible API.

Two things are here noted:

1. The sin itself, which is here particularly described by the Holy Ghost, that men might be instructed concerning the nature and the great evil of this sin of self-pollution, which is such that it brought upon the actor of it the extraordinary vengeance of God, and which is condemned not only by Scripture, but even by the light of nature, and the judgment of heathens, who have expressly censured it as a great sin, and as a kind of murder. Of which see my Latin Synopsis. Whereby we may sufficiently understand how wicked and abominable a practice this is amongst Christians, and in the light of the gospel, which lays greater and stricter obligations upon us to purity, and severely forbids all pollution both of flesh and spirit.

2. The cause of this wickedness; which seems to have been either hatred of his brother, or envy at his brother’s name and honour, springing from the pride of his own heart.

Matthew Poole's English Annotations on the Holy Bible (1683–1685) — public domain. Text from a community e-Sword/MySword module.

Vers. 9, 10. And Onan knew that the seed should not be his; and it came to pass, when literally, and it was if, i.e. whenever (cf. Ewald, Hebrews Synt., he went in unto his brother"s wife, that he spilled it on the ground (literally, destroyed to the ground), lest that he should (or, so as not to) give seed to his brother. And the thing which he did displeased (literally, was evil in the eyes of) the Lord: the word Jehovah is employed not because the writer was a late interpolator, but because the sin of Onan was an offence against the sanctity and prosperity of the theocratic family (Hengstenberg)wherefore he (i.e. Jehovah) slew him also (vide supra).

The Pulpit Commentary (1880–1919), ed. H.D.M. Spence-Jones & Joseph S. Exell — public domain. Text from a community e-Sword module.

וידע אונן, he realised this only when his father said to him that it was his task to see that his brother’s seed would be perpetuated. ושחת ארצה, he ejaculated outside her vagina, just as we explained in connection with Er.

Radak on the Tanakh — English translations: “Sefaria Community Translation” (CC0 1.0 (public-domain dedication)); “Eliyahu Munk, HaChut Hameshulash” (CC BY 4.0); “Rabbi Mike Feuer, Jerusalem Anthology” (CC BY 4.0); and “R. David Kimhi on the first book of Psalms, Translated by R.G. Finch, London, 1919” (public domain). Text via Sefaria.