As is excellently reported on by Garance Franke-Ruta in The Atlantic, this is the equivalent of "only witches float" and has been a strangely difficult-to-shake ideology for the Republican party. This despite the fact that according to research published in the American Journal of Obstetrics and Gynecology, an estimated 32,101 rape victims become pregnant each year.
James Leon Holmes, a Federal judge in Arkansas, once wrote that "Concern for rape victims is a red herring because conceptions from rape occur with approximately the same frequency as snowfall in Miami."
Which is true. As long as it snows 87 times per DAY, every day of the year, in Miami.
Update: The Onion reports Todd Akin's retraction. Heh, we wish. Warning: offensive. Like the phrase "Legitimate rape" wasn't offensive.