It's not completely implausible there is an ulterior motive. And she is well protected if there is. He, on the other hand is not.
None of us know enough about the situation to make a determination in either direction.
If the boss has known me for a long time and it seems completely out of character for me (which it would be), then maybe he talks to me about it to see WTF is up.
But some dude who just got hired? Assuming there's no record and no witnesses? You have to go with the vastly more likely case that he did it.
If that gets 1 in 10,000,000 guys unfairly fired, but gets 9,999,999 guys justly fired, that's a pretty good outcome in my book.
Is a risk-based approach less idealistic than a noble pursuit for absolute truth? Sure, but we live in the real world. Companies have to protect themselves, and there is no perfect solution for preventing sexual harassment. Education is critical, but so is a firm response.