In a somewhat exotic setting castration is a rather common sort of punishment for various sorts of offences. A promising young criminal is convicted, and subject to castration. Since there are few options for eunuchs to make a living (for instance: being kept as a servant by an eccentric master or mistress, trying one's luck in the entertainment branch of the local economy), many of them are expected to engage in prostitution. The story could describe the subject adapting to the changed circumstances, developing a new identity, both as a eunuch and prostitute, with a focus not so much on experiences, but on attitudes and mentality.

Well... strangely enough, the story I posted a while back hits on most of your points. New identiy, prostitution (kinda), focus on attitudes/mentality, etc. Mind you, it has a minor (underage) character as the protagonist. Find it at http://www.eunuch.org/Alpha/C/ea_211817a_canadi.htm .