God's view of adultery in this case seems harsh, especially since Abimelech was told that Sarah was Abraham's wife. On reading the story, I belive that Abraham more commited a sin - a sin by deception which would have endangered Abimelech to commit adultery. It seems to me, that he onus is being put on Abimelech instead of Abraham.
The lesson here to me, is once again, God's ways are surely not our ways, therefore, at times His ways may be difficult for us to comprehend.