As a defence lawyer I can say what many sexual assault files look like, pre-trial:

A woman and a man had just met at a bar or or through a friend. They had drinks, went back to one person's residence together, allegedly the man sexually assaulted her (typically involving consensual activities, transitioning into non-consensual sexual activity in one form or another by the man going "too far," etc) and then there is a police complaint. 

Then the man tells his story - often to the Police or Crown directly or through counsel - and it's the same story from start to finish, except the sexual activity is described as fully consensual. All the other details are about the same.

No other witnesses. No other evidence that isn't completely consistent with both stories. Each story sounds plausible and internally consistent.

Crown policy manual: Innocent until proven guilty. Proof beyond a reasonable doubt required. Cannot prosecute without a "reasonable prospect of conviction".

Case dropped.