There's a little debate on what Onan was being punished for, whether it was the not-impregnating-and-disobeying-the-Lord or the spilling-seed part. I incline to the disobedience explanation; as the Wikipedia entry mentions, Leviciticus's punishment for spilling seeds is very far from 'execution'.
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.