Ya think Onan was the first guy to pull out? Nahhhhhhh...
Here’s what really happened, and it has nothing to do with birth control: Context means everything, and a lack of understanding of the Torah causes much misunderstanding of its events.
God slew Onan because the man contemptuously refused to fulfill his familial responsibility under the Old Covenant. This particular practice is called levirate marriage, in which a dead man’s closest unmarried male relation (usually a younger brother, as in this case) married the widow to produce an heir for the dead man. This duty is spelled out in Deuteronomy 25:5-10 to preserve tribal inheritance rights (verse 6). Another, happier circumstance of levirate marriage is recorded in the book of Ruth, an event that eventually produced Israel’s greatest king, David (Ruth 4:17).
The problem with your reasoning is that the bible specifically said why God slew him. Why was he so significant that God “made an example of him,” well, that’s where your context matters. But what was his sin? The bible is specific.