Unsubstanciated evidence is not evidence.
He said, she said ...
Using that ‘burden of proof’ gives the Police power to make up any story they want. Would you want to go to jail, have a sex crime added to your permanent history based upon nothing more than the word of a policeman? Look at the cop wrong, drive the wrong make of car, root for the wrong team - and you too could be charged with prostitution, assault, indecent exposure or any host of crimes.
Hell, why even bother with a court system? We’ll just have the cops assign whatever crime they want, whenever they want, against whomever they want.
Prostitution rarely leaves evidence other than testimony of the involved parties. If the testimony of an undercover officer is to be universally disregarded — it would leave an unprosecutable crime.
As with all witnesses, the jury can weigh whether or not to accept the officer’s testimony as truth. If they think he is making it up, they can choose to disregard his testimony. But, his testimony is evidence, and thus can be weighed by a jury.
SnakeDoc
Unsubstanciated evidence is not evidence, it is in this day and age. Police routinely prevail in traffic court on only the word (speeding tickets). This is just a step further. This is part of the reason that many no longer trust the police.