Was Onan really struck dead by God for masturbating?

Onan's sin is most accurately interpreted as withdrawal, or coitus interruptus, as affable geek has said. But interpreting the sin as masturbation is strained even further when you consider the relevant Leviticus passages (my Wikisource translation), chapter 15, verses 16-18:

And a man which a s*m*n discharge will leave, and he bathed all his flesh in water, and he is defiled until evening. And all cloth and all leather which upon it the s*m*n discharge will be, and it will be washed in water, and it is defiled until evening. And a woman, which a man will lay with discharging s*m*n, and they bathed in water, and they are defiled until the evening.

These verses are not solely referring to nocturnal emissions, as the last verse makes clear. It nowhere prohibits masturbation, nor non-standard sex. It simply (sensibly, in my opinion) asks you to wash up afterwards.

Here is a continuation versus 19 and 24 (linked by some irrelevant stuff, same translation, my translation)

¹⁹And a woman who will be discharging, blood will be discharging in her flesh, seven days will she be in her menstrual defilement, and all that touches her will be defiled until the evening... ²⁴And if lay a man will lay with her, and her menstruation will be upon him, and he is defiled seven days, and all the couch which he shall lie upon will be defiled.

The main injunction is against touching blood. There is no special injunction against the necessarily non-procreative sex involved here.

The blood injunction is the main theme of Pentateuch law. The blood is to be drained from animals before eating, it is to be sacrificed to God, and it is not to be eaten or touched. The defiling aspects of blood over and above other discharges are repeated many times, For example, in my translation of Lamentations 4:13-15, on Wikisource:

From the sins of her prophets, the torment of her priests, that spill within her, the blood of saints. they swayed blind outside, were touched by blood, so that their clothes, no one could touch. "Away, defiled!" they called to them, "Away! Away! Do not touch". So they fled, also swayed, and said, among foreigners they will not continue to wander.

The reason for this seems to be that the blood is considered the resting place of the soul. The injunctions on s*m*n are minor by comparison. Consider Leviticus 17:11 ( again, my translation, available on Wikisource ):

Because the soul of the flesh, it is in the blood, and I will give it to you on the altar, to atone for your souls, because it is the blood, which will atone for the soul.

So on the whole, Leviticus doesn't care about masturbation (beyond cleaning up afterward), so it is inconceivable that the even more lax pre-Leviticus code would.

This answer is copied from one I posted here on the Biblical Hermeneutics SE site. That question was a response to this one, and links to several of the answers given to this question. Since it is really the same question asked in a different way on a different SE site, I think this answer should go here as well.

Here is the answer:

Onan's sin was entirely related to his refusal to perform his levirate duty.

Quickly about the other three [possibilities raised by links to answers to this question]:

1. Coitus interruptus is not masturbation. It is a (very unreliable) method of birth control. Onan was attempting not to get Tamar pregnant because he did not want to provide an heir for his deceased older brother.

2. It was not "theft of Tamar's child." Rather, it was (once again) refusal to provide his deceased brother, Er, with an heir under the law of levirate marriage.

3. Though marrying a brother's wife was normally prohibited (see Leviticus 18:16; 20:21), the law of levirate marriage was an exception to that rule. Marrying a deceased brother's wife under this law was not considered adultery or inchastity. And the rules of "inchastity" for men were rather lax in those days anyway.

The law of levirate marriage is stated in Deuteronomy 25:5-10:

5 "If brothers dwell together, and one of them dies and has no son, the wife of the dead man shall not be married outside the family to a stranger. Her husband's brother shall go in to her and take her as his wife and perform the duty of a husband's brother to her. 6 And the first son whom she bears shall succeed to the name of his dead brother, that his name may not be blotted out of Israel. 7 And if the man does not wish to take his brother's wife, then his brother's wife shall go up to the gate to the elders and say, 'My husband's brother refuses to perpetuate his brother's name in Israel; he will not perform the duty of a husband's brother to me.' 8 Then the elders of his city shall call him and speak to him, and if he persists, saying, 'I do not wish to take her,' 9 then his brother's wife shall go up to him in the presence of the elders and pull his sandal off his foot and spit in his face. And she shall answer and say, 'So shall it be done to the man who does not build up his brother's house.’ 10 And the name of his house shall be called in Israel, ‘The house of him who had his sandal pulled off.'"

Although this law was recorded several centuries after the time of Onan, as with many statutes in the Mosaic Law it was a codification of a law that was already in effect long before the time of Moses, and that was common to many ancient cultures. For more on the law of levirate marriage, see: Jewish Encyclopedia -> Levirate Marriage (Hebr. "yibbum").

The law of levirate marriage was, however, complicated in its application, and has been the subject of much debate and varying interpretation since ancient times. Having said that, here is a summary of the key events in the passage, followed by the most likely scenario of precisely what Onan's sin consisted of.

At the time of the incident of Onan in Genesis 38:6-11, Judah had fathered three sons: Er, Onan, and Shelah (see Genesis 38:1-5). Er, the eldest, had received the punishment of death from the Lord for unknown offenses. Judah then instructed his second son, Onan, to fulfill the law of levirate marriage for Er, his deceased elder brother, and provide an heir for him. Onan went through the motions of doing this, but avoided actually getting Tamar pregnant by the practice of coitus interruptus, termed in the Biblical account "wasting his s*m*n on the ground."

The text states Onan's purpose for doing this as follows:

But Onan knew that the offspring would not be his. So whenever he went in to his brother's wife he would waste the s*m*n on the ground, so as not to give offspring to his brother. (Genesis 38:9)

This was why the Lord carried out a sentence of death on Onan:

And what he did was wicked in the sight of the Lord, and he put him to death also. (Genesis 38:10)

Why did Onan marry Tamar, but then avoid getting her pregnant, and why was this an offense punishable by death in the Lord's eyes?

By going through the motions of taking Tamar as his wife, and thus appearing to fulfill the law of levirate marriage, Onan would become the heir to his deceased brother's current possessions. If he refused to fulfill his duty under the law of levirate marriage, he would not receive his brother's property. So he had a financial incentive to at least appear to fulfill the levirate law.

However, if he got Tamar pregnant, and she bore a child, especially if it were a male child, that child would inherit:

1. The primogeniture that would have gone to his father Er, Judah's eldest son, which would cause the leadership of the clan to pass to the child, and
2. A double inheritance from Judah, their father, which the firstborn son was entitled to by long custom (see, for example, Deuteronomy 21:15-17).

As mentioned above, Onan would have already received his deceased brother's property by taking Tamar as his wife. However, if he impregnated her and gave her a child, especially if it were a male child, he would lose both the primogeniture and the double portion of the inheritance from their father Judah.

A bit of simple math shows that assuming Judah fathered no more children, this would cut Onan's inheritance from two-thirds to one-fourth of his father's total wealth when his father died. And of course, it would deprive Onan of the primogeniture, or leadership of the clan, that would otherwise pass to him because his elder brother would have had no heir.

Additional source for the above: Jewish Encyclopedia -> Inheritance ("yerusbah," "naḥalah") -> Levirate Connections

The sin of Onan, then, for which the Lord punished him with death, was that of going through the motions of obeying the law of levirate marriage in order to obtain his deceased brother's property, but not actually performing his duty under that law--that of providing an heir for his brother--in order keep for himself both the primogeniture and the inheritance of a double portion of the wealth of their father Judah.

Short version: Onan made a pretense of following the levirate law, but in fact shirked his obligations to his family and clan under the law, all from motives of greed and desire for power.

The story of Judah and Tamar is an interesting one, to say the least. Judah and Tamar are both explicitly mentioned in Jesus' lineage (Matthew 1 - and Tamar is only one of four women mentioned, all of whom have sordid stories associated with them). In the end, Tamar is going to trick her father-in-law into getting her pregnant, and then avoids death by proving Judah is the father. But the question is about Onan, not Judah...

In the story, Tamar is first wed to Er, the eldest of Judah's sons. Er dies for some unmentioned reason, and Tamar is left without children. Children in that time were important both for the social support for parents in their old age, and for continuance of the family line. Being childless, the law calls for something called 'levirate marriage' - the next oldest brother is required to marry and give the widow the children. This is how Judah's second child, Onan, enters the story.

Then Judah said to Onan, “Sleep with your brother’s wife and fulfill your duty to her as a brother-in-law to raise up offspring for your brother.” But Onan knew that the child would not be his; so whenever he slept with his brother’s wife, he spilled his s*m*n on the ground to keep from providing offspring for his brother. What he did was wicked in the LORD’s sight; so the LORD put him to death also

As you can see, Onan was not masturbating. If anything, you could accuse Onan of coitus interruptus, but not masturbation.

What Onan was doing, however, was using Tamar for sex, but not giving her the child she deserved. One could call this theft - it isn't really the sex that is at issue at all. As the text clearly states, Onan did not want his deceased brother to have an heir. That would have negated his claim on the inheritance, and his new found status as eldest brother.

Interestingly, the story continues with this theme of theft - Judah refuses to allow his third son Shelah, to fufill this duty. (I mean, can you blame him? Two sons dead, and now looking at a third?) Instead of Shelah taking from Tamar, Judah is "stealing" Tamar's due. In the end, however, Tamar tricks Judah into securing her place in the line of Christ.

Just your typical Sunday School story, I know, but a fun one :)