So what you're saying is when you start doing it as a job, it takes the fun away?
Wait, were you trying to make an observation unique to prostitution? I'm pretty sure that applies to any job ;)
I assume you meant that tongue in cheek, but for the record anyway:
A) The thing "unique" to prostitution is the high degree (in terms of both frequency and severity) of force used to make people prostitutes against their will.
B) Not everyone stops liking something once they get paid for it.
C) I have read a biography of a prostitute who did get into The Life by choice, did enjoy it and all that. One thing she indicated was that clients often had trouble accepting it that she genuinely enjoyed it and were often kind of embarrassed that she achieved orgasm with them.
D) I think the difference is that sex is "supposed to be" enjoyable -- evolution designed it that way so we would be lured into reproducing in spite of the hardships and such that genuinely entails -- and work isn't usually something people assume you "should" simply enjoy. That's part of why people have to actually give advice that "you should do what you love" or "follow your bliss": Because "work" is more synonymous for most people with "drudgery" than with "pleasure".
I'm pretty sure it applies more to some jobs then others. I know you're being facetious but the main difference between prostitution and sex is that sex is generally with people you want to have sex with and prostitution is not. The difference isn't just that you're doing it for money so it suddenly becomes boring, although that can probably happen with some jobs.
So what you're saying is when you start doing it as a job, it takes the fun away?
Wait, were you trying to make an observation unique to prostitution? I'm pretty sure that applies to any job ;)