If Akin were a Democrat, none of us would have heard of this at all. Clearly, illegitimate rape is when someone lies and says that the sexual act wasn't a willing one when in fact it was at least a willing act, perhaps even an eager act or perhaps didn't occur at all.
Akin may well have his biology wrong. But clearly not all claims of rape are legitimate. The Duke Lacrosse Case is a well known example of an illegitimate rape.