[Aquinasfan]

That is a derivative argument and awful shaky ground to set up a prohibition. By that logic, one could ban cars because of the temptation to speed, especially for the young.

That's why it's paragraph #17 of a very solid natural law argument.

I'll ask you the same question. What Bible verse prohibits contraception?

Which Bible verse says that only evils that are explicitly stated in the Bible are truly evil?

Regardless, the sin of Onan is clear enough. At least it was to all Christians prior to 1929.

[sauropod]

The sin of Onan was that he disobeyed God's command to procreate if memory serves. He spilled his seed on the ground to prevent procreation. It was not the act itself that was wrong, it was the intention behind the act. Trying to build a logical bridge between this and contreceptives is akin to trying to build that famous bridge to the 21st century.

Contraceptives are not intrinsically evil. I'm not buying your argument.