It is often incredibly difficult to prosecute sexual crimes like this that don't involve a violent struggle. There is almost never any evidence that can distinguish beyond a reasonable doubt non-consensual sex from consensual sex. It is almost always the word of one party against the word of another. However, like the previous examples of police impropriety, that has no bearing on the validity of the accusations. Victims should feel welcome to come forward with their experiences regardless of the likelihood of their accusations resulting in a conviction.

Absolutely agree with the last statement, I was mostly thinking from a prosecutors perspective. Is there any other crime that can only be prosecuted by the criminal admitting what they did?

Certain crimes have multiple degrees which hinge on intent which often can’t be proved without a confession.