Partisans always think people should believe members of their group. The rest of us know that real victims need support, not belief. In seeking the truth, you have to stay open to the possibility that any one, everyone, or no one may be telling the true story. People don't have to be lying to be wrong; humans make mistakes. That's why the presumption of innocence should be given to the accuser as well as the accused.
ETA: The most recent example of presumed guilty: University of Chicago Gets Its Own 'Rapist List'
If you truly always believe the woman: hoax hate crimes.
ETA 2: If someone tells you to believe automatically, hide your wallet.