>>18123676
Because... uhhh
I don't think an honest reader of Genesis would come to the conclusion that Onan's sin was masturbation. It's that there a very specific expectation to raise a child for a dead brother (who would inherit said brother's property). Onan basically just fucked his brother's wife, pulled out of her pussy, and thought he could keep his brother's house/money etc.
I think because it's such a bizarre concept only found in Israelite culture that speed-readers like Aquinas just couldn't grasp it. He just went "wow... he cummed on the floor instead of in her pussy... THAT must be it!!!"
But husbands cum all over their wives all the time instead of in their pussy and they don't drop dead... neither does anyone not retarded consider such a thing "masturbation"