He wants sex, not the babies, can't blame the guy. But there's not yet any reliable method of contraception back then other than not having sex at all. Have to settle with second best. :P
The sin of Onan was failure to fulfill his obligations in a Levite-marriage. His new wife needed children to help take care of her when she got older (there was no Social-security or retirement programs in those days), but he refused in one of the more publicly-known and disrespectful ways.
For whatever reason this was misinterpreted as Onan's sin being ejaculation without the intention of conception (either through masturbation or the use of protection). The point of needing to secure financial safety for a loved one is the biggie.
What's the word that originated from the Old Testament character Onan? However, unlike its modern meaning of masturbation, it originally meant ejaculating outside the vagina.
Are you satisfied now?Onanism, right?