How do we understand the Bible verse Genesis 38:8-10?
In those days, if your older brother died and he and his wife were childless, it meant that she would become destitute. (After all, she would not have a son to care for her.) So, in order to preserve your older brother’s lineage and to ensure that his wife would have someone to care for her, the next oldest son had to ensure that his sister-in-law would give birth to a child.
Onan, recognizing that any child she had wouldn’t be his, knew he wouldn’t see any benefit from getting her pregnant. (In fact, it could
hurt his inheritance.) So, he didn’t really want to fulfill his obligation.
However, rather than just saying ‘no’, he actually had sex with her, but in a way that made it impossible for her to conceive. Think about it for a second: he actually had the gall to lay with his sister-in-law, but not for the reason of giving her a child!
That’s the sin of Onan: using sex purely for pleasure, and not for the possibility of conception.
Could Onan get to the heaven?
Just like any of us, if he repented before dying, then yes.
And why shouldn’t we use onanism for contraception?
Well, first off, I hope there aren’t any brothers-in-law having sex with their widowed sisters-in-law.
Secondly, though, the answer is “because we see why it was wrong for
him, and for the same reason, it’s wrong for
us to do.”