EWTN Catholic Q&A
Question from Karen on 09-28-2017:

The church's prohibition on contraception is based essentially on Onan in Genesis.

If God did not want man to 'spill his seed' why did God design males such that they have nocturnal emissions?

Granted, it is not a deliberate act by men. However, the nocturnal ejaculation is the result of brain activity focused on producing pleasure, not children. This bodily function starts well before the age of marriage.

God did that. God designed the male brain and anatomy so that there would be ejaculation outside of intercourse in marriage. Why? Why did God do that?

Answer by Judie Brown on 09-29-2017:


As Father Steven Torraco has pointed out, the nocturnal emissions of the male happens naturally and is not in any way to be confused with sinful actions such as the use of contraception or masturbation or others types of sexual sins.

Father Brian Harrison, a noted theologian, explains the sexual sin of Onan in a way that I think will help you see why your question is divorced from logic because there is a huge difference between what God has designed for man and the sexual sins committed by man. Harrison writes

"...when sexuality is treated in its most sublime character -- i.e., marriage as a sacred mystery symbolizing God's covenantal love with His people -- the Bible's allusions to the conjugal act are predominantly indirect and allegorical. The implications of this for Genesis 38:9, wherein Onan's sexual act is described in starkly explicit terms, are clear.

"It should be remembered also that we are here dealing here with a culture which so abhorred that other form of "wasting the seed" -- the homosexual act -- that it prescribed the death penalty for this offence. In the light of this and the other factors we have considered, I submit that it would be not only exegetically unwarranted, but quite anachronistic, to suggest that the Genesis author, in line with the 'political correctness' of late twentieth-century Western liberalism, would have taken a relaxed, indulgent view of Onan's method of preventing conception -- his "spill[ing] the seed on the ground." We should note also the parallel between the description of homosexual acts as a "wicked" or or "abominable" thing in the Leviticus texts and the similar qualification of what Onan did in Genesis 38:10."

The complete article is at https://www.catholicculture.org/culture/library/view.cfm? recnum=2940