I would argue that there is a difference between having an undercover officer posing as a prostitute, and having a virtual prostitute. In the former case, by paying the officer, you are -- in a sense -- engaging a prostitute. In the latter case, you are paying for what amounts to a virtual service.
... a virtual service that the perpetrator believed was real.
Past sting operations in the narcotics space have involved fake drugs---flour in plastic bags. Perpetrators who either bought or attempted to steal the fake drugs were convicted. One can debate the morality of stings in general, but I think the question of intent vs. reality is a settled question regarding legal precedent.
