Originally Posted By: Jose Gros
The Good Book states that penalty was for "spilling semen into ground", am I right?

Not exactly, no. That is just a description of what Onan did so as not to impregnate his dead brother's wife. The penalty in Onan's case was for practising what was in effect contraception by premature withdrawal in order to deny his brother a continuing line of children, probably because they would have competed with his own children by his wife. It is not a law saying that if you spill your seed on the ground God will punish you, like for example the law about not coveting you neighbour's wife or stealing. It is a story about something that happened. The only law involved in this story was the law of Levirate marriage - that was the law he was breaking.