Really? Not to be interrogative, but could you please provide a source that backs this up? As far as I know, the tradition interpretation has always been that the sin was coitus interruptus, hence the term "Onanism."
Is Genesis 38 authoritative enough?
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 when he went in to his brother’s wife he spilled the semen on the ground, lest he should give offspring to his brother. 10 And what he did was displeasing in the sight of the Lord, and he slew him also.
Well, no, because I could just as easily bold the unbolded part and make my case for the opposite. I'm asking more for an authoritative interpretation.
You could, but your case would be weak. The entire passage centers around Onan going in to his brother's wife for a particular purpose, not "just because". The spilling of his semen was the way he obstructed that purpose. There's no reason to believe that God's hot displeasure lashed out against Onan simply for spilling his seed in and of itself given how many times the levirate marriage angle is underscored in the passage.
If there are other places in the OT where God prohibits withdrawal in the Mosaic law, or in some other stories, I'd be happy to reconsider this interpretation.
I agree that "the traditional interpretation" has to do with declaring spilling the seed an abomination in all its forms, but I'm not convinced of this just based on Scripture itself. This is not because I'm eager to "get away with sin", but rather I think it is a more convenient interpretation. The "wrongness" of spilling the seed is the one application that can be extended to all people by reference to this story. It's much more difficult to explain why it was OK for you to sleep with your sister-in-law back in the day, both in the sight of society and in the sight of God, but not anymore.
"Traditional interpretations" of matters related to sexuality are sometimes a stretch. For example, the "traditional" interpretation of I Cor 7 is that St Paul was positively commanding abstinence from sexual relations for married couples for certain periods dedicated to prayer: the canons related to Lenten discipline cite this passage as the basis for imposing abstinence on married couples. And yet, St Paul is basically saying the exact opposite: "if you both want to, maybe that would be a good idea, but only temporarily, and definitely make sure to come back together again", not "Lent is only fifty days long, deal with it" or "Plan your amorous activities on Mondays, because God doesn't want to deal with it any other day of the week".