Quiddity
UndertheInfluenceofGiants
There's more to the story of Onan, though: in that case, God specifically commands Onan to impregnate his brother's widow, so it would seem to me that it's defiance of God that's the problem there, not the specific method that Onan used to defy God. I mean, Lot's wife was cursed for disobeying God by turning her head, but we don't infer from this that neck movement is bad in general.
The penalty for refusal to continue the family of your brother was public humiliation.
Deut. 25:
8Thereupon the elders of his city shall summon him and speak to him. If he persists in saying, I do not want to marry her, 9* his sister-in-law, in the presence of the elders, shall go up to him and strip his sandal from his foot and spit in his face, declaring, This is how one should be treated who will not build up his brothers family! 10And his name shall be called in Israel, the house of the man stripped of his sandal.
It was not death.
Unless of course one wishes to say God made an exception here because it was God directly giving the order? But that opens up a whole can of worms.