sobino,
I think from your earlier posts you are on the right track.
There are a few obvious things to consider about Onan, and the bible.
Although contraception according to medical scholars is quite rampant at this time and especially later (Roman period), (most kinds) – barier intestinal, plant (pharmika) – yet it rarely is mentioned in the bible, and when mentioned is never described in any detail with the exception of Onan.
As Daniel Marsh partially points out, Jewish idiom usually hides the name under a sanitized expression which not all would get until old enough – eg: nakedness refers to any sordid sexual sinful act, to uncover one’s mothers nakedness is to commit incest from son to mother — and so forth. It is a geralized expression, which for some reason is not used about Onan. Normal sexual relations is referred to as “going in” to a woman and “knowing” her.
The very fact that the act of Onan is described in such detail means that the author was going beyond the normal taboo in describing sexual acts to give the extent of the reason Onan’s sin was particularly condemnable. Perhaps the original author could only point to the contraceptive nature of the act as explicable of the death – since “wet” dreams and other such phenomina are mentioned elsewhere as being merely a soruce of “unclenliness” for a day – not a wicked act.
The next thing I would note is that although many others violated levirate law, they were not killed for it. Consider, even Judah broke Hebrew custom by marrying a pagan woman in the first place; as there was undoubtedly a closer kin to whom he owed marriage at this time. But instead Judah married a Pagan woman of the faimily of a man whom he had befriended. This very act would later be the major downfall of the Israelite people, and would be likened to idolatry – the intermarrying of Pagans and accepting their religious practices.
Not a single one of these brothers of this pagan woman makes it into the later geneology of the tribe of Juda – God cuts all three off – but he only executes two, the eldest and second eldest who acted wickedly. The youngest son presumably is not executed because he does not spill the seed - (sperma) in Greek – as he does not marry at all.
The reason for the act being so dispicable as deserving death – though other acts of contraception do not merit death explicitly – in terms of God’s eyes – has two possible sources, one is the source of example. As the Egyptians would be slain in the Red sea, and the unfaithful Israelites in the desert – as an example to future generations – a token execution foreshadowing and revealing God’s will.
But this incident also has to do with the promise of the messiah, as you are probably aware – Jesus is from the tribe of Judah and had Onan and Er succeeded is preventing Tamar from ever having children, God’s promise of salvation handed down through a geneology (in the way he revealed he would do it) – could not have come to fruition through Judah. The sin of Er and Onan then has a magnitude beyond the sin of contraception, in that the sin itself was aimed at destroying God’s entire plan for our salvation.
Explore these thoughts a little, and it will become apparent in the Catholic worldview why contraception is the specific sin being condemned here – and not merely the breaking of a custom which doesn’t carry the death penalty in Levirate law – as already pointed out.
However if you are interested I could send you a link to some thoughts by a biblical scholar. I would copy and paste here but not sure if it is frowned on.