Was Onan really struck dead by God for masturbating?

Historically, Onan in Genesis 38 has been [accused of masturbation](http://en.wikipedia.org/wiki/Onan) , and for this supposed transgression was struck dead. The term Onanism (or in German __Onanie__) is still used to describe this practice. For example: - Jerome write: >But I wonder why he the heretic Jovinianus set Judah and Tamar before us for an example, unless perchance even harlots give him pleasure; or Onan, who was slain because he grudged his brother his seed. Does he imagine that we approve of any sexual intercourse except for the procreation of children? - Clement of Alexandria, while not making explicit reference to Onan, similarly reflects an early Christian view of the abhorrence of spilling seed: >Because of its divine institution for the propagation of man, the seed is not to be vainly ejaculated, nor is it to be damaged, nor is it to be wasted. To have coitus other than to procreate children is to do injury to nature Was Onan's sin really masturbation?