You took that verse completely out of context. Nice try.
There is not a single instance of real intercourse (marital and in the procreative form) which is condemned in Scripture.
How's that for context?
There is not a single instance of falsified intercourse (non-marital/ not in the procreative form), which is blessed or pronounced pleasing to God in Scripture.
How's that for context?
Moreover, the sin of Onan was not merely that he would not reproduce with Tamar. There was no "death penalty" simply for the refusal to have children with the brother's widow. The penalty for such refusal --- for turning way from the obliations of levitate marriage --- was a public shaming (Deut. 25). The refused woman could take the man to the city gates, strip his sandal off of his foot, and in the presence of the elders, spit in his face.
But the Genesis episode with Onan doesn't say he merely declined to reproduce. It wasn't merely what Onan *didn't* do. It says that what he DID do --- his action --- was evil in the eyes of the Lord: "Therefore the Lord slew him, because he did a detestable thing."
You can't get a much stronger message, Scripturally, than that.
The clincher is that every single Christian denonimation saw it that way until the abandonment of sexual morality in the 20th century. No Christian denomination in history approved of contraception until 1930, when the Anglicans broke ranks and officially iapproved of it at their Lambeth Conference in 1930.
So the whole weight of testimony for 5,000 years, BC, AD and into the 20th century, is: yes to God's design for real, lovemaking-lifemaking, honest marital intercourse; no to any sex act that intentionally negates that design.