What should we do to the rapist? Should we execute him by ripping his body apart? No? Then why should we do that to a baby conceived in rape? Abortion does nothing to help or comfort the woman. It just makes a terrible situation worse by further traumatizing her. Genuine compassion provides help for the woman and her baby, and deals with criminals justly.
See Ryan Bomberger's Radiance Foundation for a very positive perspective on this. It is better to light a candle than to curse the darkness.