Atheist answer: nothing. Birth control wasn't an option during the Bronze Age, nor was it an option 2,000 year ago. The Bible no more addresses birth control than it does computers and airplanes.
The fundy answer is a little longer.
Man was commissioned by God “to be fruitful and multiply” (Genesis 1:28). Marriage was instituted by God as a stable environment in which to produce and raise children. Sadly, children today are sometimes considered a nuisance and a burden. They stand in the way of people’s career paths and financial goals, and they “crimp your style” socially. Often, this type of selfishness is at the root of contraceptive use.
Does anyone, anywhere, not have children for these reasons? I know some people that don't have children and don't want to, myself included. None of us think of children as "nuisances" or "burdens". "I can't afford a child" is not the same thing as regarding a child as a burden, it's a realistic understanding of personal finances. (What is up with religious conservatives who condemn people on welfare one second, then condemn people for not having more children the next? Do they not see the connection?)
As for "crimping your style" socially: where? Not having children crimps my social style because everyone I know has children. They can't engage in adults only social activities, and I don't have any interest in Chuck E. Cheese, so you can see the problem. But I'm selfish, you see.
Contrary to the self-centeredness behind some birth control usage, the Bible presents children as a gift from God (Genesis 4:1; Genesis 33:5). Children are a heritage from the Lord (Psalm 127:3-5). Children are a blessing from God (Luke 1:42). Children are a crown to the aged (Proverbs 17:6). God blesses barren women with children (Psalm 113:9; Genesis 21:1-3; 25:21-22; 30:1-2; 1 Samuel 1:6-8; Luke 1:7, 24-25). God forms children in the womb (Psalm 139:13-16). God knows children before their birth (Jeremiah 1:5; Galatians 1:15). Yes, yes, yes, god likes the bebbes. Well, not all the children. And where else would god "form" children?
The next paragraph is actually the most reasoned interpretation of the Onan story I've seen. I've got to give them props in this case.
The closest that Scripture comes to specifically condemning birth control is Genesis chapter 38, the account of Judah's sons Er and Onan. Er married a woman named Tamar, but he was wicked and the Lord put him to death, leaving Tamar with no husband or children. Tamar was given in marriage to Er's brother, Onan, in accordance with the law of levirate marriage in Deuteronomy 25:5-6. Onan did not want to split his inheritance with any child that he might produce on his brother's behalf, so he practiced the oldest form of birth control, withdrawal. Genesis 38:10 says, “What he did was wicked in the LORD's sight; so He put him to death also.” Onan's motivation was selfish: he used Tamar for his own pleasure, but refused to perform his legal duty of creating an heir for his deceased brother. This passage is often used as evidence that God does not approve of birth control. However, it was not explicitly the act of contraception that caused the Lord to put Onan to death; it was Onan’s selfish motives behind the action.It is important to view children as God sees them, not as the world tells us we should. the world tells us to view children as burdens that crimp our style? srsly? Having said that, the Bible does not forbid contraception. Contraception, by definition, is merely the opposite of conception. uh, yeah. It is not the act of contraception itself that determines whether it is wrong or right. As we learned from Onan, it is the motivation behind the contraception that determines if it is right or wrong. If a married couple is practicing contraception in order to have more for themselves, then it is wrong. if you're actually not having children because they would keep you from buying the latest fashions, then you should not have children. why would anyone want to subject a child to that? If a couple is practicing contraception in order to temporarily delay children until they are more mature and more financially and spiritually prepared, then it is perhaps acceptable to use contraception for a time. maybe. but get with the baby making soon, you selfish bastards. that church isn't going to tithe itself. Again, it all comes back to motivation.
The Bible always presents having children as a good thing. The Bible “expects” that a husband and wife will have children. well, duh. when the bible was written there was no reliable method of birth control. so, if you were having sex, babies would result. The inability to have children is always presented in Scripture as a bad thing. There is no one in the Bible who expressed a desire not to have any children. i don't see anyone in the bible expressing a desire to use the internet, is using the internet a bad thing? At the same time, it cannot be argued from the Bible that it is explicitly wrong to use birth control for a limited time. All married couples should seek the Lord’s will in regards to when they should try to have children and how many children they seek to have. that's basically a nonanswer. "well, the bible doesn't really say . . . go pray!"