To be consistent, then God should instantly zot anybody who uses contraception in any form, even Onan's method?
I got to put my interpretation here fwiw. Judah's eldest son, who was the "first born" and therefore heir to be head of the family, died childless. The levirate custom, which was practiced in Canaan in those days, required that the dead man's brother take his widow and attempt to give her offspring. While this might have been a form of "social security", its main purpose was to have the new child "step into the shoes" of the dead brother.
The Scripture says, "Onan knew the seed would not be his", so he withdrew prematurely, "lest he give seed to his brother." He would have been reasoning this way: "If I take my brother's widow, and she never gets pregnant by me, then I inherit the pot and my own kids, by my own wife, will get it after me. But if Tamar gets a child, then he gets the pot and my own kids play second fiddle to HIM!"
True, but tough tacos. Onan was second fiddle to start with, but this got him kicked out of the orchestra.
Yeah, the real lesson in Genesis 38 is DO WHAT GOD TELLS YOU.