That is not exactly true, the process and biology are quite complicated. So if the female in question did in fact have a loving husband her chances of being impregnated by a rapist are considerable less than a more or less celibate single female. It turns out that not all sperm make a mad swim for the egg, some hang back, block and attack "foreign" sperm. I offer this as just one instance of how complicated the process really is.
As a doctor, I can assure you that there is no magic protection in a woman's body to decide that some sperm are from a rapist and barred from the egg, while others are from a loving husband and have access. Akin's statement about this was just medically wrong. Check the CDC page, and the studies cited there.