If the victim says that the accused did X, Y, and then Z, that is a statement of fact about what happened. If the victim then says that he doesn't think that X, Y, and Z constitute a crime, that is a statement about the law..which might just be incorrect.
* except for strict liability crimes like speeding
However, if the "victim" says there was no rape, and has never said so, then the whole premise, including both the intent and the act, is unseated.
Disclaimer: I am no lawyer, and do not know much about this specific case. These are just my intuitions about rape laws. I also believe that if the laws, in fact, are much different from these ideas in spirit, they are SORELY off-base.
I am simply saying that in a criminal case if the only evidence presented is the accusation of the prosecution, especially if the supposed victim proclaims they are not a victim of the alleged crime, then I will find it hard to convict if I'm on that jury. Regardless of what the law says or the alleged intent of the accused.
In cases that any law can provide a conviction with no evidence other than an accusation from the state then that's an extremely bad law and should be stricken.
Plus your stabbing example doesn't explain anything since you would have indeed physically harmed someone which is against the law in most jurisdictions. Considering your rape/not-rape example, a better example is that you would be arrested for thinking about stabbing someone in the arm as you are placing your hand on that arm.
If I were on the jury for either of these examples, supposed victim says no crime committed and the only evidence is the accusation from the state? There's no way you could convince me to vote to convict.