"8 Then Judah said to Onan, “Go in to your brother's wife and perform the duty of a brother-in-law to her, and raise up offspring for your brother.” 9 But Onan knew that the offspring would not be his. So whenever he went in to his brother's wife he would waste the semen on the ground, so as not to give offspring to his brother. 10 And what he did was wicked in the sight of the Lord, and he put him to death also."
That doesn't leave much doubt about WHY he was supposed to have sex with her, or what his rebellion against God was. I fail to see how this applies to a husband and wife being intimate without creating a baby.
The purpose of any contraception is not to “raise offspring”. There is nothing there that applies uniquely to Onan and Thamar, but not to a regular marriage.
The levirical marriage, as cited earlier says that the surviving brother has to marry the widow. While the purpose of the marriage is indeed to raise a son in the deceased’s name, that is not that different from any other marriage, which is always connected to the overriding commandment to “procreate and fill the earth”. We saw from by emphasis earlier that Thamar was a full-fledged wife, not a sperm recipient. Now, implicit in marriage is to have sex. If, for example, Onan and Thamar did not produce an offspring despite trying, there would be no transgression.
Onan’s behavior was exactly contracepted sex and he was punished precisely for contracepting.