Q: Is it scripturally wrong for a married couple to purposefully prevent the conception of children? A lot of people have told me that “God slew Onan because he was not keeping the law, (providing children for his dead brother's wife) not for simply “using birth-control.” If that is so, why did God not slay Shelah, Onan's younger brother, for not marrying Tamar and begetting her children? Was the “wicked” thing Onan did (Gen. 38:9-10) using sex in a non-fruitful fashion?
A: No, it is not scripturally wrong for a married couple to choose not to have children, nor to limit the number of children they choose to have. The Bible does not prohibit one to prevent conception if one so chooses.
While God slaying Onan is sometimes used as an example, it is not a legitimate example. The reason God killed him is not because he used birth control. God killed Onan because of his motivation, which was his refusal to raise up a seed for his deceased brother. As for Shelah, the reason God did not kill him is because he was never actually married to Tamar, as Onan was, and therefore, he did not have a physical relation with Tamar. The sin did not lie with Shelah but it lay with Judah, who, as the patriarch, refused to give Shelah to Tamar. It is not an issue of Shelah's refusal but Judah's refusal to give Shelah to Tamar. Onan's sin was not “using sex in a non-fruitful fashion” as such. His sin was in simply refusing to allow a child of his to be reckoned as the seed of his brother.