Er, if you have a justice system that requires proof of innocence rather than proof of guilt, that is a bigger problem than any line-drawing around the boundaries of rape.

If there is not consent (or capacity for consent) at the time of the act, it's rape. In any reasonable justice system, the proof problems that come up, in certain situations, with that clear definition weigh in favor of the accused, since they make it difficult to prove that sex occurred without consent.