Genesis 38:9

Onan married Tamar, but he was not willing to provide an heir for his brother. He had sexual relations with Tamar, but he did not fulfill his duty to his dead brother (38:8).

This story describes the Lord as the sovereign judge who ended the lives of Judah's wicked sons.

The story of Judah and Tamar is a carefully placed break in the larger story. It shows what was happening in Judah’s family. Judah would later become an important leader. The story also shows the early stages of the family mixing with the people of the land. This helps explain why God sent the family to Egypt (chapters 39–47).

The Egyptians kept themselves separate from other peoples (43:32). Because of that, the Israelites were more likely to keep their own identity in Egypt than in Canaan.

Judah said unto Onan . . . marry her, and raise up seed to thy brother--The first instance of a custom, which was afterwards incorporated among the laws of Moses, that when a husband died leaving a widow, his brother next of age was to marry her, and the issue, if any, was to be served heir to the deceased (compare Deu 25:5).

Here is, 1. Judah's foolish friendship with a Canaanite-man. He went down from his brethren, and withdrew for a time from their society and his father's family, and got to be intimately acquainted with one Hirah, an Adullamite, Gen 38:1. It is computed that he was now not much above fifteen or sixteen years of age, an easy prey to the tempter. Note, When young people that have been well educated begin to change their company, they will soon change their manners, and lose their good education. Those that go down from their brethren, that despise and forsake the society of the seed of Israel, and pick up Canaanites for their companions, are going down the hill apace. It is of great consequence to young people to choose proper associates; for these they will imitate, study to recommend themselves to, and, by their opinion of them, value themselves: an error in this choice is often fatal. 2. His foolish marriage with a Canaanite-woman, a match made, not by his father, who, it should seem, was not consulted, but by his new friend Hirah, Gen 38:2. Many have been drawn into marriages scandalous and pernicious to themselves and their families by keeping bad company, and growing familiar with bad people: one wicked league entangles men in another. Let young people be admonished by this to take their good parents for their best friends, and to be advised by them, and not by flatterers, who wheedle them, to make a prey of them. 3. His children by this Canaanite, and his disposal of them. Three sons he had by her, Er, Onan, and Shelah. It is probable that she embraced the worship of the God of Israel, at least in profession, but, for aught that appears, there was little of the fear of God in the family. Judah married too young, and very rashly; he also married his sons too young, when they had neither wit nor grace to govern themselves, and the consequences were very bad. (1.) His first-born, Er, was notoriously wicked; he was so in the sight of the Lord, that is, in defiance of God and his law; or, if perhaps he was not wicked in the sight of God, to whom all men's wickedness is open; and what came of it? Why, God cut him off presently (Gen 38:7): The Lord slew him. Note, Sometimes God makes quick work with sinners, and takes them away in his wrath, when they are but just setting out in a wicked course of life. (2.) The next son, Onan, was, according to the ancient usage, married to the widow, to preserve the name of his deceased brother that died childless. Though God had taken away his life for his wickedness, yet they were solicitous to preserve his memory; and their disappointment therein, through Onan's sin, was a further punishment of his wickedness. The custom of marrying the brother's widow was afterwards made one of the laws of Moses, Deu 25:5. Onan, though he consented to marry the widow, yet, to the great abuse of his own body, of the wife that he had married, and of the memory of his brother that was gone, he refused to raise up seed unto his brother, as he was in duty bound. This was so much the worse because the Messiah was to descend from Judah, and, had he not been guilty of this wickedness, he might have had the honour of being one of his ancestors. Note, Those sins that dishonour the body and defile it are very displeasing to God and evidences of vile affections. (3.) Shelah, the third son, was reserved for the widow (Gen 38:11), yet with a design that he should not marry so young as his brothers had done, lest he die also. Some think that Judah never intended to marry Shelah to Tamar, but unjustly suspected her to have been the death of her two former husbands (whereas it was their own wickedness that slew them), and then sent her to her father's house, with a charge to remain a widow. If so, it was an inexcusable piece of prevarication that he was guilty of. However, Tamar acquiesced for the present, and waited the issue.

Go in unto thy brother's wife. Although no law had hitherto been prescribed concerning brother's marriages, that the surviving brother should raise up seed to one who was dead; it is, nevertheless, not wonderful that, by the mere instinct of nature, men should have been inclined to this course. For since each man is born for the preservation of the whole race, if any one dies without children, there seems to be here some defect of nature. It was deemed therefore an act of humanity to acquire some name for the dead, from which it might appear that they had lived. Now, the only reason why the children born to the surviving brother, should be reckoned to him who had died, was, that there might be no dry branch in the family; and in this manner they took away the reproach of barrenness. Besides, since the woman is given as a help to the man, when any woman married into a family, she was, in a certain sense, given up to the name of that family. According to this reasoning, Tamar was not altogether free, but was held under an obligation to the house of Judah, to procreate some seed. Now, though this does not proceed from any rule of piety, yet the Lord had impressed it upon the hearts of man as a duty of humanity; as he afterwards commanded it to the Jews in their polity. Hence we infer the malignity of Onan, who envied his brother this honor, and would not allow him, when dead, to obtain the title of father; and this redounds to the dishonor of the whole family. We see that many grant their own sons to their friends for adoption: it was, therefore, an outrageous act of barbarity to deny to his own brother what is given even to strangers. [139] Moreover he has not only shortened his brother concerning the right due to him, but he rather spilled seed on the ground than to raise a son in his brother's name.

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.

And Onan knew that the seed should not be his,.... Should not be called a son of his, but a son of his brother Er; this is to be understood only of the firstborn; all the rest of the children born afterwards were reckoned the children of the real parent of them; this shows this was a custom in use in those times, and well known, and was not a peculiar case:

and it came to pass, when he went in unto his brother's wife; to cohabit with her, as man and wife, he having married her according to his father's direction:

that he spilled it on the ground, lest he should give his seed to his brother: lest his brother's wife he had married should conceive by him, and bear a son that should be called his brother's, and inherit his estate; and this is the sin, which from him is called Onania, a sin condemned by the light of nature, as well as by the word of God, and very prejudicial to mankind, as well as displeasing to God, as follows:

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.

About this time, i.e., after the sale of Joseph, while still feeding the flocks of Jacob along with his brethren (Gen 37:26),
(Note: As the expression "at that time" does not compel us to place Judah's marriage after the sale of Joseph, many have followed Augustine (qusaet. 123), and placed it some years earlier. But this assumption is rendered extremely improbable, if not impossible, by the fact that Judah was not merely accidentally present when Joseph was sold, but was evidently living with his brethren, and had not yet set up an establishment of his own; whereas he had settled at Adullam previous to his marriage, and seems to have lived there up to the time of the birth of the twins by Thamar. Moreover, the 23 years which intervened between the taking of Joseph into Egypt and the migration of Jacob thither, furnish space enough for all the events recorded in this chapter. If we suppose that Judah, who was 20 years old when Joseph was sold, went to Adullam soon afterwards and married there, is three sons might have been born four or five years after Joseph's captivity. And if his eldest son was born about a year and a half after the sale of Joseph, and he married him to Thamar when he was 15 years old, and gave her to his second son a year after that, Onan's death would occur at least five years before Jacob's removal to Egypt; time enough, therefore, both for the generation and birth of the twin-sons of Judah by Thamar, and for Judah's two journeys into Egypt with his brethren to buy corn. (See Gen 46:8.))
Judah separated from them, and went down (from Hebron, Gen 37:14, or the mountains) to Adullam, in the lowland (Jos 15:35), into the neighbourhood of a man named Hirah. "He pitched (his tent, Gen 26:25) up to a man of Adullam," i.e., in his neighbourhood, so as to enter into friendly intercourse with him.
Gen 38:2-5
There Judah married the daughter of Shuah, a Canaanite, and had three sons by her: Ger (ער), Onan, and Shelah. The name of the place is mentioned when the last is born, viz., Chezib or Achzib (Jos 15:44; Mic 1:14), in the southern portion of the lowland of Judah, that the descendants of Shelah might know the birth-place of their ancestor. This was unnecessary in the case of the others, who died childless.
Gen 38:6-10
When Ger was grown up, according to ancient custom (cf. Gen 21:21; Gen 34:4) his father gave him a wife, named Thamar, probably a Canaanite, of unknown parentage. But Ger was soon put to death by Jehovah on account of his wickedness. Judah then wished Onan, as the brother-in-law, to marry the childless widow of his deceased brother, and raise up seed, i.e., a family, for him. But as he knew that the first-born son would not be the founder of his own family, but would perpetuate the family of the deceased and receive his inheritance, he prevented conception when consummating the marriage by spilling the semen. ארצה שׁחת, "destroyed to the ground (i.e., let it fall upon the ground), so as not to give seed to his brother" (נתן for תּת only here and Num 20:21). This act not only betrayed a want of affection to his brother, combined with a despicable covetousness for his possession and inheritance, but was also a sin against the divine institution of marriage and its object, and was therefore punished by Jehovah with sudden death. The custom of levirate marriage, which is first mentioned here, and is found in different forms among Indians, Persians, and other nations of Asia and Africa, was not founded upon a divine command, but upon an ancient tradition, originating probably in Chaldea. It was not abolished, however, by the Mosaic law (Deu 25:5.), but only so far restricted as not to allow it to interfere with the sanctity of marriage; and with this limitation it was enjoined as a duty of affection to build up the brother's house, and to preserve his family and name (see my Bibl. Archologie, 108).
Gen 38:11
The sudden death of his two sons so soon after their marriage with Thamar made Judah hesitate to give her the third as a husband also, thinking, very likely, according to a superstition which we find in Tobit 3:7ff., that either she herself, or marriage with her, had been the cause of her husbands' deaths. He therefore sent her away to her father's house, with the promise that he would give her his youngest son as soon as he had grown up; though he never intended it seriously, "for he thought lest (פּן אמר, i.e., he was afraid that) he also might die like his brethren."