It's not about whether the law never believes the accusations of one over the other. The burden of proof is with the accuser, and it's for them to prove beyond reasonable doubt the nature of their accusation. Therefore a 'not guilty' verdict does not necessarily mean that the accuser was lying; it can often mean that they didn't put across enough of a case to prove their accusation.
It can sometimes be that to a greater or lesser degree that neither party was lying, or that both parties were lying, which is different from your automatic black-and-white assumptions of 'he's not guilty therefore she's lying...'
Quite. Which is why I thank you for helping me go through my argument, and showing me its weaknesses.