system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why are rape cases so hard to prosecute?

Is it lack of evidence? victim blaming? one persons word versus another? why is rape harder to prosecute than other crimes?
gpt
With most crimes, the act itself is criminal - so any substantial evidence suggesting the act took place can potentially result in a criminal conviction.

In the case of rape or sexual assault, however, the act itself is not criminal by nature. The same act can be legal or illegal and it's largely dependent on how the other person reacts psychologically to the sexual activity/advances. 

It's a case where if the sexual partner wants to engage in those activities (or is open to sexual advances) then the act is not a crime, but if the sexual partner does not want to engage in those activities (or is not open to sexual advances) and an individual pursues those actions/advances anyway, then it becomes a crime. 

Proving that the sexual partner did not want to have sexual relations with that individual at that time of the offense is the challenge. You basically have to prove the victim's state of mind at the time of the offense, either by convincing the court that the victim was too psychologically impaired to consent to those activities or by convincing the court that the victim did not provide consent and the offender continued anyway despite the victim's unwillingness to engage in such acts.

Without audio/video evidence, it's a lot of he said / she said about the event itself and/or the relationship the two had (if any) before and/or after the event.