The incest justification for abortion is just a distraction. Of the million or so US abortions every year, how many involve pregnancies resulting from incest? I'd be surprised if there are as many as ten in a year. Incest doesn't involve an unidentifiable stranger. The father is known, and his responsibility can be proved by DNA testing.
Forcible rape by an unknown rapist is certainly possible, and using a weapon to "persuade" the victim to submit is still forcible even if no bodily injury is inflicted. However, Akin's use of the term "legitimate" was intended to distinguish rapes that happened from rapes that are claimed but never occurred. The Duke lacrosse team and a disbarred prosecutor showed that.