Pulling out. The Bible doesn't get too much into birth control... it's clearly pro-populating but, back when it was written, no one really anticipated the condom or the sponge, so those don't get specific bans.
But... pulling out does. One of the most famous sexual-oriented Bible verses... the one that's used as anti-masturbation rhetoric... is actually anti-pulling out.
It's Genesis 38:9-10: "Onan knew that the offspring would not be his; so when he went in to his brother's wife, he wasted his seed on the ground in order not to give offspring to his brother. But what he did was displeasing in the sight of the Lord; so He took his life also." (full story here)
Yep -- pull out and get smote. That's harsh.