First, "Legitimate" rape. I think he was speaking not about the legitimacy (if there is such a thing) of rape, but the legitimacy or truthfulness of the report. This distinction doesn't get Akin out of trouble, though. Because the comment, however you phrase it, reveals more than a casual belief that many rape reports are untrue. I don't doubt that there are SOME false rape reports, but to assume that a significant portion of them are false sounds to me like misogyny.
The other, or "medical", half of the statement reveals either abysmal ignorance or conscious falsehood, again based on misogyny.
He says "doctors have told him". What kind of "doctors"? Doctors of podiatry? Doctors of Theology? Doctors of Veterinary Medicine? Doctors of Mechanical Engineering? Witch doctors?
Even if his alleged informants are doctors of medicine, the plural term "doctors" may mean that somewhere in the United States (or maybe in the world) there are as many as two of them who are so ignorant or careless or malevolent as to tell him that.
All of the above possibilities assume that Akin himself isn't just issuing a rape report without legitimacy.