You have to prove them guilty beyond reasonable doubt (surely you’ve heard this phrase before), which means 100% proof

That's not true, because "beyond reasonable doubt" is usually interpreted very differently.
You do not need 100% proof. Otherwise convictions would never happen.

To stay with sexual assault cases, the defendant could always argue that Consensual Non Consent (Couples setting limits beforehand, and then after that acting like one party does not consent) was part of their kink, and happening at the time.
No eyewitnesses, Video, or even written statements could completely rule out the possibility of (increasingly elaborate) consensual non-consent.

And the same goes for anything else. Bank robbery? Well, I was hired to test their security. No Idea why they are now fucking me over, they probably don't want to pay.

Murder? I was told the weapon was a toy, and we were acting out a stage-play!

anyway, you just need to proof beyond doubt that seems reasonable. Most of my examples above wouldn't meet that requirement, depending on further circumstances.

There are 2 burdens of proof in trials, 1 for civil trials, and 1 for criminal trials. Civil trials require a burden of proof "beyond a reasonable doubt" and it is much lower than the burden of proof required for criminal trials, which is "beyond a shadow of a doubt". The burden of proof you are describing for Kevin Spacey's criminal trial is actually "beyond a shadow of a doubt", which essentially requires reliable eyewitnesses or a smoking gun, as they say.

That said, Spacey also defeated a civil trial in the US last October for a different set of accusations, so there is that 🤷

I can't claim to know the truth in he said/he said situations like these, but common sense would indicate that there's probably some truth to multiple accusations of impropriety. Victims often don't opt to speak out publicly and go to court unless they think they can win and scam artists are rare.