The whole Bible is a refutation and condemnation of Birth control by any means, whether they are natural, or unnatural, or artificial. The Catholic Rhythm Method, endorsed by the Catholic Humanae Vitae Encyclical kills babies just as efficiently as Condoms, or any other method, device or poison. The Catholic Church is totally wrong in splitting hairs that way, and they violated 2000 years of Church teaching in order to do it. They call it a providential decision. Meaning they didn't want to risk emptying the Church of members if they took a hard line on killing babies by any means.
Now understand this: Birth Control is worse than abortion, and abortion is worse than infanticide, and infanticide is worse than murder, so go figure. With each step up the line the baby has less chance to live.
Now Onan was executed by God and sent to Hell, for taking innocent life NOT NOT NOT for violating the Leverite Law, or disobeying God in any way. Our God is a just God, and He does not kill idly. The punishment for violating the Leverite Law was a small fine and public humiliation by the woman slapping the man across the face with her sandal in front of the whole church congregation.