Indeed - I don't allege anything at all in the example in terms of who is truthful or what really happened. My point is merely that, since both stories are plausible, consistent, and supported by external evidence (i.e. friends of the two people also agreeing that they remember the two people leaving the bar together at a certain time, etc.), it is impossible for the Crown to see a reasonable prospect of conviction, so they decline to charge. 

It's not about whether the accusation was "substantiated" or "unfounded" or any other word. It's just a basic fact that the Crown is faced with proving someone's guilt beyond a reasonable doubt, and they have nowhere near enough evidence to get there. Remember, "Beyond a reasonable doubt" means proof to an extremely high likelihood of guilt; proof to a virtual certainty.

That is the reason most sexual assault files don't make it trial. It's not politics. It's just plain old insufficient evidence to meet the high standard of proof.