The verse Funditards cite as to why they think that Birth Control is against God's will is 38:8-10.
To recap this story. Er the son of Judah was offensive to Yahweh. Doesn't say how he was offensive to Yahweh, but he was, so God killed him. Then Judah directed his brother Onan to marry Tamar, Er's widow. Well, old Onan didn't want to father kids on his dead brother's wife, but he had no problem having sex with her. He just pulled out just before the climax. Well, this offended God, too. And God like totally smited that fool.
ANYWHO- the part of the same story the Funditards don't want to talk about. Tamar thought that she should be married to Judah's third son, but Judah wasn't having any of that. This chick is like cursed. So Tamar disguised herself as a hooker, and in a fashion Jerry Springer would be proud of, seduced her father in law. She asked for his seal and rod as payment.
Well, low and behold, she finally did get knocked up - with twins- and Judah wanted to totally burn her alive for it, not realizing he was the father. So much for the "sanctity of the unborn" crap they try to claim when opposing abortion. She had a child outside marriage, she was totally getting burned.
Only when she pointed out the kid was his, did he relent.
Now, this is actually kind of biblically important, because one of these two kids was Pharez, who became an ancestor of David and eventually Jesus.