In my recent conversation with Dennis Prager, the subject of masturbation came up. 

Prager said that since masturbation is nowhere condemned in the Old or New Testaments, this serves as evidence that masturbation is not a sin. 

This is what is called an argument from silence, where a conclusion is based upon silence or lack of evidence. Such arguments aren’t always illegitimate, but they are notoriously weak and inconclusive. The Bible also does not condemn insider trading and chattel slavery, for example, but Prager would condemn these as sins.

Prager also responded preemptively to something he suspected I might bring up—the sin of Onan:

Judah took a wife for Er his firstborn, and her name was Tamar. But Er, Judah’s first-born, was wicked in the sight of the Lord; and the Lord slew him. 

Then Judah said to Onan, “Go in to your brother’s wife, and perform the duty of a brother-in-law to her, and raise up offspring for your brother.” But Onan knew that the offspring would not be his; so when he went in to his brother’s wife he spilled the semen on the ground, lest he should give offspring to his brother. And what he did was displeasing in the sight of the Lord, and he slew him also (Gen. 38:6-10).

This passage involves the Hebrew custom of levirate marriage, according to which, if a man died childless, it was his brother’s duty to marry the widow and father children who would be legally regarded as sons of the dead man. It is called levirate marriage since levir is the Latin term for brother-in-law, and the woman’s brother-in-law is the biological father of the children.

Onan did not wish to father children for his dead brother, but he did want to have sexual pleasure from the widow, so he practiced coitus interruptus to avoid inseminating her—allowing him the pleasure of sex with the woman but preventing him from fathering children for his brother.

This passage has been used by many Christians as a prooftext against deliberately rendering the sexual act infertile, which includes masturbation.

However, Prager made the case that Onan was punished not for “spilling his seed” but for violating the levirate marriage custom.

At the time of my discussion with Prager, I wasn’t equipped to respond to this objection in the heat of the moment, and so I addressed his objection from a natural law perspective. Namely, we can know by reason what sex is for (union and procreation), and to deliberately thwart the end of sex is a perversion. 

While it can be tricky to discern the nuances of what was in the mind of a biblical author writing 3,000 years ago, if I could engage Prager on this again, I would make these points:

1. Prager Is at Odds with Other Jews

Prager is at odds with the respected Jewish commentary, Bereishis: Genesis which states:

[Onan] misused the organs God gave him for propagating the race to unnaturally satisfy his own lust, and he was therefore deserving of death (5:1677).

2. The Penalty Is Far Less Severe in the Mosaic law

If Prager was correct and Onan was struck dead by God for refusing to give offspring to his deceased brother’s wife, why isn’t the death penalty applied to other cases where a man refuses to fulfill the duty of a brother-in-law?  The penalty announced by the Mosaic law isn’t death but merely public humiliation. If a man refuses to perform the duty, 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.” And the name of his house shall be called in Israel, “The house of him who had his sandal pulled off” (Deut. 25:9-10). 

The fact that the Mosaic law required public humiliation for failing to fulfill the responsibility—but Onan was put to death—indicates that Onan was guilty of more than just failing to fulfill the responsibility. There was something immoral about the sexual acts that he performed with the woman. 

Thus, the text says that “what he did [i.e., the sex acts with coitus interruptus] was displeasing in the sight of the Lord.” It does not say “what he did not do [i.e., give children to his brother]” was what earned him death. The latter failure would only have warranted public humiliation according to God’s law through Moses. The way the text presents matters, it’s what Onan did do, not what he didn’t do, that caused his death.

3. The Use of Graphic Language

Hebrew contains several ways of referring to the sexual act—to “know,” to “go in to,” and to “lie with.” The first of these is the most decorous and normally refers to lawful, wholesome sexuality (e.g., “Adam knew Eve his wife,” Gen. 4:1). The latter two are more blunt and can refer to both lawful and unlawful sex.

In Genesis 38, we go beyond these simple descriptions, and we are told the precise details of what Onan did: “When he went in to his brother’s wife he spilled the semen on the ground.” This unexpected intrusion of the graphic details of what Onan did calls attention to the specific thing that he did wrong. As Fr. Brian Harrison notes:

If the inspired author, while knowing the same historical facts, had evaluated them in the way most modern exegetes would have us believe he did (with moral indifference toward Onan’s contraceptive act as such), we would expect quite different wording. “Spilling the seed,” being irrelevant to the author’s interest and purpose on that hypothesis, would not even have been mentioned. Instead, we would expect to be faced with an account stating more discreetly that, even though Onan took Tamar legally as his wife, he refused to allow her to conceive, so that God slew him for his “hardness of heart,” his pride, or perhaps his avarice (in wanting his brother’s property to pass to himself and his own sons) (The Real Sin of Onan).

4. Natural Law

We also should consider what happened from the perspective of natural law—that is, what a reasonable person would conclude based on human nature.

One way of applying this to the current situation is to simply ask, “What are human genitals supposed to do? What’s their function?”

The answer is obvious. They have two functions: eliminating waste (urine) and facilitating reproduction (directly, through the transmission of semen, and indirectly, through uniting the spouses). These are their obvious, proper uses.

Therefore, if you’re doing something else with your genitals—or if you deliberately keep them from fulfilling their proper uses, as Onan did—then you are misusing them by definition.

5. Protestants Should Agree With Catholics