Gen. 38:9-10 But Onan knew that the offspring would not be his; so whenever he lay with his brother's wife, he spilled his semen on the ground to keep from producing offspring for his brother. What he did was wicked in the Lord's sight; so he put him to death also.

Was God unfair to Onan? Skeptic Dennis McKinsey regards it as "almost too ridiculous too discuss" that God would kill Onan for refusing to impregnate his sister-in-law. Indeed it would be, for a skeptic hiding in his 20th-century air-conditioned house and living in comfort and in no need of corporate survival benefits. In fact Onan committed no less than four despicable sins, any one of which was worthy of God's judgment [Hamilton's Genesis commentary, 435]:

1. Onan refused to carry out his responsibility as the brother of the deceased. For Onan's sister-in-law, whose original husband had died, to have a child was the only way to ensure an inheritance in the family and survive. In this sort of social situation, ancient protocol demanded that the nearest male kin to the dead man provide his services (so to speak) to produce an heir. This concept was later institutionalized in the Deuteronomic law as levirate marriage (Dt. 25). Ruth and Boaz were an example of this type of arrangement, and Joseph and Mary may have been as well.
2. Onan not only refused, but repeatedly refused. This is a point Jonathan Kirsch misses in his book The Harlot by the Side of the Road -- which goes to show what happens when a journalist tries to understand the Bible on his own terms and study. The verse as rendered above carries the connotation and syntax fairly well: whenever Onan lay with Tamar -- not just once, but every time. Onan was a "repeat offender" and had plenty of chances to fulfill his obligation. God was merciful and gave him plenty of leeway before doing away with him!
3. Onan not only repeatedly refused, but pretended that he wasn't refusing. If Onan was not willing to take part, all he had to do was say so. It would have been an insult, and he would have had to suffer disgrace (cf. Num. 27:8-11 -- though these practices were widespread in the ANE) for being such a jerk and not helping out a helpless widow, but it was still his option. Instead, he decided to go for having his cake and eating it too, by having the pleasure of conjugal relations with Tamar, while avoiding the obligation -- how selfish can you get!? Even more selfish -- that leads to sin #4:
4. Onan was therefore setting it up so that he would get the big bucks and leave Tamar penniless and helpless. If Tamar had no heir born to her, guess who got the family's inheritance? Give you a hint: His name started with "O"!

Bottom line: Any skeptic who feels sorry for Onan the Barbarian just doesn't get the point. They're thinking time out of mind, yet again!