Taking any form of birth control is not operating (walking) by faith. It is sin...the works of the flesh. In other words you think you know better than Gods plan for your life.
Ok given your quote about Onan, as I understand it he wasn't punished because he withdrew but for the reasons why.
I'm sorry but I don't agree with you in that taking any type of birth control is a sin. I am 48 and my wife is 45. We have 4 kids, the eldest is 20 and youngest 13. My wife takes birth control, so according to your comment she is sinning and therefore I am as I'm complicit with her.
So to me your comment is in a sense judgmental on me and my wife and anyone who uses some kind of birth control. We agreed on birth control, one that after much research and thought and prayer was one that as Papa Zoom correctly states above is not abortive. So we are happy with our choice.
The truth is that every life from the beginning of time has been given by God, regardless of the circumstances.
And I think God gives life even when birth control is used.
Furthermore I beleive every aborted child is in heaven. I agree with Papa that when the sperm and egg meet and fertilisation takes place then life is formed. Now whether that life is aborted medically or via abortive contraceptive that life, that child is in heaven.
You mention works of the flesh and birth control is a work of the flesh.
Galatians 5:19-21
Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God.
Can't see birth control in the above so maybe you could expand why you think it is a work of the flesh.
Also we know
Galatians 5:22-25
But the fruit of the Spirit is love, joy, peace, longsuffering, kindness, goodness, faithfulness, gentleness, self-control. Against such there is no law. And those who are Christ's have crucified the flesh with its passions and desires. If we live in the Spirit, let us also walk in the Spirit.
My wife takes birth control yet I find her manifesting the fruit of the Spirit, she is an amazing Godly woman. Passionate about Jesus and not afraid to tell others about Jesus. She walks according to the Spirit and not the flesh.
But as I read what you say she is walking according to the flesh and is sinning.
If I've got you wrong please let me know.