Lest I be accused of misrepresentation, I do want to clarify a few things about Onanism.
Onanists do understand that Onan was deliberately disobeying God. They do not simply conclude that the spilling itself was the whole of Onan’s transgressions.
The implications of what they believe are quite staggering. While most don’t believe there are any reasons to prevent pregnancy, there are some who will allow it. However, because of their interpretation of these verses, they often assert the only moral way to prevent pregnancy is complete abstinence, even in marriage. I know a man who has gone twelve years without intercourse because he refused his wife’s request to have a vasectomy. He considers it his cross to bear. It’s almost like a badge of honor to him. I think it’s a shame. I think it is atrocious that he has allowed his wife to withhold from him (sin) for this long without approaching her as a brother in Christ or as a Godly husband concerned with his wife’s soul.
They also don’t go officially by the name “Onanist”, I just coined the term to describe those who hold to this particular application of the Onan story. So don’t go pointing a finger and yelling “Onanist!” at them. They probably wouldn’t understand anyway.