At that point you'd have to make a judgement call. If someone is being publicly accused of a sexual assault which he denies too place but doesn't show any interest in launching a libel suit, that makes me rather suspicious. On the other hand, if he files a libel suit, or if criminal charges are filed against him, it seems reasonable to step back and let the courts decide what happened.

I'm not saying that I have a good answer for all possible situations, only that I know that I'd start by listening to both sides.

The company may face pressure for me to act. Do I throw my employee under the bus, on the basis of my judgment of her statement?

You're really asking two questions there. Is it reasonable to fire someone based on your judgement of what happened? Quite possibly. Is it reasonable to fire someone based on public pressure alone? I would say absolutely not -- but I come from an academic background, and I feel the greatest moments of academic history have been when universities have defended their faculty against public opinion. Most CEOs would probably throw an employee under a bus at the first hint of public pressure.