Gen 38:8-10 is a traditional prooftext against "artificial" contraception (and masturbation). But what exactly is Onan's sin? According to one interpretation, "wasting seed" is a grave or mortal sin. To put this in Thomistic natural law terms, "wasting seed" thwarts the natural telos of seminal fluid. There are, however, problems with that interpretation:
i) V9 adds a qualification. Not merely "wasting his seed", but for a particular reason. So intent is a necessary condition.
ii) If wasting seed per se is sinful, then that drastically restricts licit sexual intercourse. Having intercourse with a postmenopausal wife is "wasting seed". Having intercourse with a manifestly pregnant woman is "wasting seed".
Even during ovulation, only one, or rarely, two sperm will be able to fertilize the ovum. Most sperm are "wasted". They never reach the goal.
iii) Ironically, if wasting seed per se is sinful, then this is a prooftext against "natural family planning". The husband is deliberately wasting his seed by intentionally limiting intercourse to infertile periods during his wife's cycle. By design, the couple is frustrating the natural teleology of semen. A calculated effort to subvert and circumvent the biological goal of seminal fluid. What could be more wasteful than that? So this is a highly counterproductive text to defend "natural family planning" as the licit alternative to "artificial" contraception.
I had always thought Onan's sin was "early withdrawl" and that not in and of itself. He was supposed to be fulfilling a Levrite responsibility and enabling his brother's widow to have a child. Instead he repeatedly enjoyed the liason but pulled out in time to insure there would not be an heir.
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