As I recalled the judge stated what he believed most likely took place regarding how the woman's pants were removed. In the circumstances there are only two sensible possibilities: that the woman consented to the removal, or that she was unconscious when it happened. If the former took place, no sexual assault occurred despite the immoral circumstances. If the latter took place then a sexual assault did occur. So it's up to the Crown to prove that the latter is actually what happened, and they couldn't do that. 

Again, take a look at the bigger picture. The issue of consent is there to PROTECT people from sexual assault, and overall it surely has that effect. True, in some cases the issue of consent will be difficult or impossible to judge, but we're still further ahead for having it as a legal requirement. And people do have power in the situation: don't drink to the point of stupefaction. You are less likely to be victimized and more likely to recall it if you are.