You're right, what happened after the first trip in the bathroom shouldn't matter... But it does. In a situation of he-said, she-said, with little or no physical evidence of what went down, you have to rely on all the contextual clues to decide what really happened.
If, after the first trip to the bathroom, she had left the party with her friend, told her friend what happened while being visibly upset, went to the hospital to be checked out and called the police - what would have happened? They would have been arrested and charged and there would have been a good chance of conviction.
Instead, her behavior the rest of the night is not really consistent with someone who just suffered a traumatizing sexual assault at the hands of multiple men. I really want to believe her and don't doubt that she was not happy with what was going on but I am having a hard time understanding her subsequent behavior - and I am not even on a jury where I am required to be convinced beyond a reasonable doubt.