Presumption of innocence is an entirely legal concept that means you are legally innocent until proven legally guilty. Without a finding of fault the law cannot be used to punish. That is all it is there for - to determine whether there is a legal basis for punishing someone.
But it doesn't change facts. It doesn't mean factually innocent until proven guilty. Presumed legally innocent is an entirely separate concept.
People never want to believe that their idols, heroes, charming community figurehead, or friendly neighbour could be capable of monstrous acts. They want to believe the people who do monstrous things look like monsters all the time. It makes them feel safer.
Even on the occasion that a sexual assault is caught on film or camera or witnessed, perpetrators are still found legally not guilty or argue it was consensual (sometimes convicted regardless, sometimes not). It is ridiculously difficult to prove, to a legal standard, with our cultural baggage and collective denial on how sexual abuse occurs, that an offence took place. But that doesn't alter reality.
Very, very few sexual abusers ever admit to it, even in the face of overwhelming evidence, because there is such stigma attached. They want to believe for themselves that they are not monsters either.
Court proceedings for sexual abuse aren't scientific fact-proving exercises, it is a legal undertaking that does not even have anything to do with "justice". Somebody is not innocent just by virtue of never having been successfully prosecuted, and nobody is required to pretend they are (defamation laws not withstanding).
I realise the world feels nicer and safer if sexual predators looked like easily identifiable monsters and all cases of sexual assault could easily be proven either way. But the world isn't that nice, safe, predictable place, and that is not how it works. People who seem wonderful do terrible things that sometimes do not come packaged up with "proof" that forces us to accept what's happened.
People are welcome to shield themselves from reality using denial, but I'm not required to join them in their denial.
Personally, I find it thoroughly credible that children would have spent a very long time denying anything bad happened to them, maybe also believing it was a relationship and they were special, before eventually many years later coming to understand as adults what truly happened to them and eventually being able to accept - to themselves - calling it what it was. That's a very common, normal reaction to sexual abuse.
Victims of sexual abuse use denial to protect themselves just as much as anybody else.