I think the story of Onan is an indictment of both birth control and masturbation. The bible seems to cast the same eye on either practice.
Sheesh, mangeorge, you have a bible with eyes?!? :eek: I know some people have Talking Book Bibles, because they keep telling me what their Bible “says.” 
While I’ve tried to say that with a bit of wittiness, there’s a quite serious point behind it. It’s vitally important to distinguish between the words in the Bible and our understanding of “what it obviously means.”
And the story of Onan is not, directly, a condemnation of birth control, masturbation, or even coitus interruptus except in one particular circumstance, regardless of what the nuns or the Sunday School teachers may have told you. Israelite law, codified in the Mosaic Law, required (as did many cultures of the Middle East) that if your brother died childless, it was your obligation to take his wife, father a son on her, and raise that boy to adulthood – but it would not be your own son, it would offically be his, entitled to his share of the family inheritance. Later generations regarded Onan with particular shock because Er was the firstborn son of the patriarch Judah; his son would have been the hereditary head of the tribe. Onan screwed Tamar (Er’s widow) but pulled out to avoid fathering a child that would inherit the double share of the inheritance to which Er and his son would be entitled – and which would pass to Onan as eldest surviving son if Tamar did not have a child by either Er or him acting in Er’s stead. And Tamar’s future, as a widow in that culture, was dependent on being taken care of by her son when he grew to manhood.
Whatever God may think of masturbation or birth control, including coitus interruptus, this story says nothing directly about them. It’s describing a case where a man, in addition to getting a little sex off his brother’s widow, screws her in quite a different sense by refusing to take the action by which her future would be assured, because he’s greedy for what her son would get.
But using it to scare teenage boys about their sexuality is typical of the “let’s use the Bible as a boogeyman” crowd.
Gentle writings don’t use words like “abomination”, kiddo. 
Onan’s sin was in putting his seed where it didn’t belong, which means anywhere but into his (or his brothers) wife.
You’re right about one thing though, I’m no expert on the bible. I have heard “christian” people argue Onan as an indictment of homosexuality.
And I have no problem with birth control or masturbation. Or gay marriage. Any of these, involving consenting adults, is simply none of my business.