If Onan was struck down for wasting his seed, why is NFP OK?

A Protestant is asking me, “if Onan was struck down for wasting his seed, why is it ok for a married couple practicing natural family planning to have relations during the wife’s infertile time; isn’t that wasting the man’s seed?”

How do I respond to this?

Onan spilled his semen on the ground in order to render the act sterile (cf. Gen 38:9). His sin was not the waste of semen – semen is often wasted in the sense that, during relations, its presence does not always result in pregnancy. Onan’s sin was his acting out in such a way to intentionally sterilize the act.

This is very different from NFP which never attempts to render an act sterile – every act remains open to the possibility of procreation. NFP simply seeks to regulate pregnancy by observing abstinence during the woman’s fertile period.

Suggested reading:
Onan’s Real Sin
What is the moral difference between contraception by artificial means and NFP?

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