First of all, let’s get straight my case in posting this: it is NOT my intention to undermine or weaken the support for the Catholic doctrine on contraception, which, as a Catholic, I fully profess along with the whole of Catholic doctrine. Rather, I want to show that invoking “the sin of Onan” when dealing with contraception issues provides no real support for the Catholic position, but rather shows that the speaker has not bothered to interpret a biblical verse by taking into account, at the very least, the previous two verses, as we will see below.
To note, Humanae Vitae explicitely states from where the support for the Catholic doctrine comes: natural law as interpreted by the Church Magisterium:
The Church, nevertheless, in urging men to the observance of the precepts of the natural law, which it interprets by its constant doctrine, teaches that it is necessary that each and every marital act (“quilibet matrimonii usus”) remains ordered (“destinatus”) per se to the procreation of human life. (Humanae Vitae 11)
Having said this, let’s turn to the subject at hand. It seems to me that the interpretation of the Onan story (Gen 38: 8-10) that is most frequent among practicing Catholics is that God stroke Onan to death because of just the fact that “he wasted his seed on the ground”. I will call this interpretation “B” for “biological”.
Indeed, the passage says: “What he did greatly offended the LORD, and the LORD took his life too.” (Gen 38: 10)
The point is, however: what exactly is “what he did” that “greatly offended the LORD”? And there are two possible answers.
The answer from interpretation B is that “what he did” refers specifically and only to the fact that “he wasted his seed on the ground” (Gen 38: 9). Everything else in Gen 38, 9 is just accessory information, useful for understanding the context but irrelevant to the gravity of the sin.
To this interpretation and answer, I offer below an alternative interpretation, which in a fit of brilliant inspiration I will call “A”. In this interpretation, the answer to the question above is that “what he did” refers to the whole situation described by Gen 38: 9. In other words, that in order to understand what was “what he did” that “greatly offended the LORD” we have to take into account ALL the information provided by Gen 38: 9. Let’s then do that.
But first, let’s focus briefly on Gen 38: 8. Was Judah’s command to Onan morally and/or legally binding? Though this was a pre-Law situation, we can infer from the prescriptions in Dt 25: 5-10 that a refusal by Onan to take Tamar as wife would have resulted only in a brief and slight public humiliation (Tamar would have taken his sandal and spit in his face), without even lasting economical consequences. So, Onan was basically morally and legally free to not marry Tamar. (Which is logical, because after all, what if the deceased brother had a terrible taste for women, and not just, or even mainly, at the physical level?) This point is not central to the gravity of the sin in interpretation A, but just adds to it.
Turning now to Gen 38: 9, it clearly states the reason why Onan “wasted his seed on the ground”: “to avoid giving offspring to his brother”, because he “knew that the offspring would not be his”. Specifically, in Law terms which can be safely assumed to reflect the situation in Onan’s time as well, because he knew that “the firstborn son she bears shall continue the name of the deceased brother” (Dt 25: 6).
Thus it is most clear that the reason driving Onan’s contraceptive behaviour was of a permanent, not transient, nature. He was not holding off having offspring until he would be in a better economic situation, or have a bigger tent, or whatever. Rather, he was determined to NEVER, EVER have offspring with Tamar. Let’s now look at this situation from Tamar’s viewpoint.
First, she had been cheated by Onan into her present condition. Because the reason why Onan had been able to marry her was exclusively because of his pledge (implicit in the passage) to engender offspring of her, the first of which would preserve his deceased brother’s name. Was that reason important to Tamar? It most definitely was, as we know from the rest of ch. 38 to what lengths Tamar went afterwards in order to have offspring that would preserve Er’s name.
But most importantly, she was facing the prospect of not being EVER able to have offspring at all, the worst possible disgrace for a woman in that society. She had basically been reduced to the condition of a sex slave, a living sex doll, a condition that, as I said before, was going to last for as long as Onan lived. Because in that society the wife was not able to initiate the procedure for divorce or even ask for it.
Thus, the only event through which Tamar could possibly become liberated from that unspeakable condition of indignity into which she had been cheated was Onan’s death.
Therefore, in interpretation A “what Onan did” that “greatly offended the LORD” was not just a one-time or even a one-year practice of contraception. It was his act of reducing Tamar for ever, by cheating and against her will, to a state of sex slavery involving the worst possible fate for a woman in that society: the complete absence of offspring. That unspeakable assault on Tamar’s dignity was probably, in the mindset of that society, a sin at least as bad as killing her.