So, if I understand you correctly, it is a sin to intentionally have a barren marriage?
I'm not arguing against fertility per se, I'm pointing out that Genesis 38 does not necessarily mean what you say it means. If you wanted to argue that contraception violates the general principles that is laid down in scriptures, that is quite a different story.
Besides, if Genesis 38 meant what you think it means, Onan and most boys wouldn't have made it past the age of 14, let alone make it to marriage.
It is a sin to have intentionally barren intercourse. That is, to choose against the natural God-designed fertility of marital acts.
"Besides, if Genesis 38 meant what you think it means, Onan and most boys wouldn't have made it past the age of 14, let alone make it to marriage."
You don't know what I think it means. It does not mean that masturbation calls for the death penalty . (!!)
You're not just arguing with me: you're arguing with all Christian history until 1930. Biblical and historic Christianity taught that Onanism (intentionally wasting the seed) is a sin. Contracepting --- impairing the natural fertility of --- the sexual act is a sin. Christianity did not, and does not, teach that it is a capital crime.