A few comments...
I am sorry, but I don't see any signs of a tongue-in-cheek attitude in that movie.
I don't know about "tongue in cheek", but the movie certainly does not make the audience think that being a prostitute is a way to riches and fun. I can think of at least three different ways which the exact opposite messege, in fact, is being delivered:
1). THE CHARACTERS EXPLICITLY DENY HER PRSOTITUTION-TO-RICHES CASE IS TYPICAL:
For instance, Robert's character asks her friend (another prostitute) if she actually know anybody who "made it" by being a prostitue. The friend replies, "Cindi-f***en-rella!", that is, admitting that only happens in a fairy tale.
Near the end of the movie, Gere's character leaves her--after he makes sure that she has enough money to go back to school and not having to be a hooker anymore. She is happy, but says sadly to her friend: "I want the fairy tale".
2). THE ENTIRE RELATIONSHIP BETWEEN GERE AND ROBERT'S CHARACTERS IS BASED ON THE FACT THAT SHE IS NOT REALLY A PROSTITUTE, BUT MERELY A "CAPTURED INNOCENT":
This goes together with the fairy-tale idea: Cinderella is raised from the ashes of the kitchen not because kitchen maids regularly date princes, but because she isn't REALLY a kitchen maid, but, rather, noble-born.
Same thing in the movie. The relationship between Gere's and Robert's character only works because he obviously does NOT have sex with her, like he would with a regular prostitute. Why? Because he senses the "princess" (innocent, virginal, "good" woman) under her "prostitute" exterior.
To add to this, the movie makes it clear that Robert's character is "new in the biz", naive and innocent; that she and her friend are, in fact, unsuccesful prostitutes who do not have many clients at all; and that she refuses her friend's suggestion of getting a pimp, which is something a "real" prostitute (or kitchen maid) would surely do, but not a "good" woman like Robert's character.
3). HOW "REAL" PROSTITUTES ARE TREATED IS EXPLICITLY SHOWN IN THE MOVIE.
How is a movie with an ATTEMPTED RAPE scene of Roberts' character--by the boorish lawyer (player very well, by the way, by Jason Alexander) could possibly be seen as a recommendation of prostitution?
Of course the white knight (Gere) comes in in the critical moment and saves her, but it's obvious that he saves her for the same reason he falls for her in the first place: that she is not a REAL prostitute.
All this, this, of course, in addition to the fact that the movie is (to repeat) openly fiction and obviously a remake of Cinderella, so you would have to be a total idiot to rely on it for guidance on real-life prostitution even if it DID glorify fairy-tale-land prostitution--which it doesn't.
To give you another example, there are actually a few loons out there who were so taken by "Star Wars" that they seem to actually believe Luke Skywalker & co. are (were?) actual people, and that it IS possible to build light-sabers if they try hard enough.
This despite the fact that THAT movie, too, is a modern remake of Snow White with the sexes reversed (with poor Alec Guinness as Obi-Van Kenobi, the fairy godmother), and that the movie even starts, obviously deliberately, with a written legend out of a fairy tale: "A long time ago in a galaxy far, far away...".
In both cases, so what? The fact that somebody can be so stupid as to rely on such fiction as a guide to the world is hardly the movie's fault. "Pretty Women" didn't promote prostitution any more than "Star Wars" promoted building light sabers in your garage, as at least one idiot has a web page claiming that is what he's trying to do.
Could you please be specific and point out examples of the movie Pretty Woman trying to let its viewers know that it is only putting on an act for entertainment.
Er, the part that it is FICTION? That it was not advertised as a documentary? That it is not billed as "based on a true story", as most Hollywood movies which have even the most tenuous connection to real events claim to be? That its commercials, reviews, and teasers almost invariably referred to it as "A Cinderella story?"
How much more "out of your way" can you go to make clear a movie is NOT really real-life than to openly and repeatedly say (correctly) that it's a retelling of a fairy-tale--in fact, to make the fairy-tale point the "hook" that is supposed to get you to watch the movie in the first place?