Jayman

It's not about birth control, it's about levirate marriage as King Arthur said earlier. Tamar's husband Er died (38:7) and therefore Onan, Er's brother (38:3-4), had to fulfill the duty of a brother-in-law (38:8). Levirate marriage is explained in Deuteronomy 25:5-10. The Sadducees also challenge Jesus about marriage at the Resurrection and refer to levirate marriage in Mt 22:23-33/Mk 12:18-27/Lk 20:27-40. The Sadducees say, "Moses told us that if a man dies without having any children, his brother must marry the widow and have children for him."

Gringo

Perhaps Onan was motivated not to fulfill his Levitical duties not for a dislike for his sister but for his brother. The child would not be his, but instead would be for a remembrance to his brother who was wicked in the sight of the Lord. Onan's wickedness was more likely deceiving Judah his father into thinking he was fulfilling his obligation, for the penalty for the refusal to pregnate one's widowed sister-in-law is mush lighter than death, see Deut. 25:5-10. In addition, although Shelah did not fulfill this obligation, he received no blame because he wasn't commanded to impregnate his sister-in-law.

Grumpy

Rather than saying, "If it's about anything..." I should have said: "If I had to pick between masturbation or birth control, I'd say the story is about birth control." Yes, it's about obeying the laws of levirate marriage. But it also resembles a cautionary tale about coitus interruptus. Begin Monty Python singing here.

Egoinos

I always thought Onan's sin was coitus interruptus. I mean, if he had actually refused to have sex with Tamar, then he wouldn't have "spilled his seed", would he? It strikes me that he didn't mind the sex, but didn't want the kid - hence the spillage.

9 But Onan knew that the offspring would not be his; so when he went in to his brother's wife he spilled the semen on the ground, lest he should give offspring to his brother.

Well, he seems to have not wanted any child he gave to Tamar to be his brothers' - he wanted the child to be known as his son, so he made sure she would not conceive.

--Egoinos--

Grumpy

Suffice to say that the deed was coitus interruptus, as opposed to masturbation (and I'll be darned if I can figure out how anyone read that into the text in the first place). But the sin was plain disobedience of the law of levirate marriage.