Onan was told by Judah to produce an offspring for his brother. Since Onan didn't want to do this, he practiced a primitive form of birth control with his brother's wife. He was put to death because of he didn't fulfill his duty as a brother.
http://biblegateway.com/passage/?search=Genesis%2038:8-10&version=NIV
Topics: Onan, Spilled His Semen On The Ground, Spill Seed
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