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.