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.
That being said, Onanism is defined as both masturbation and coitus interruptus.
Again, I am not arguing against contraception as a historical Christian prohibition; I'm just saying Genesis 38 is not a good example. Matthew Henry's commentary on Genesis 38 points out that Onan's sin is what I showed it to be, not a general prohibition against contraception. I'm not saying that he was for contraception, just that Genesis 38 is about something different.
Again, if you want to argue the fertility route that contraception is wrong, that is a different argument.
Chapter and verse please, Mrs. Don-o?