"Onan's act almost prevented the line from which the Messiah will come (or has come, if you are Christian). That was the crime worthy of the severe penalty."
Onan didn't know about that, and so his crime was no better or worse than any other person who either (A)Practiced onanism or (B)Failed to perform his levirate duty. If all he were guilty of were (B), then he only merited the quite mild punishment presribed in Deut. 25.
Historically, Christians have always identified his crime as onanism -- until the 20th Century Protestant contraceptors came along and re-interpreted the passage to make it comatible with the recently invented Protestant doctrine endorsing contraception.
Exactly. You might be interested in the link in Post #20, btw.
But he didn't know about Deut. 25., either. For that matter, it's not clear what Onan did or did not know, except that he was supposed to conceive children by Tamar. (Indeed, if we go by the laws of Deut., what should be Abram's punishment, for sleeping with Hagar, even with Sarai's permission?)
Furthermore, following this, Tamar conceived twins by her father-in-law Judah. The punishment for this at the time, per 38:24, was burning. However, once she prove that Judah was the father, the penalty was waived.
Okay, granted, Judah may have just been saving his butt and his progeny. Still, that which was considered wrong was made right by who was involved. Certainly, then, something normally acceptable may have been made unacceptable by what the situation, as well.