In NONE of those places, I think you mean. I mentioned a town and two cities that wear clothes ubiquitously, compared to Ponyville's one small town.
Just because we do not see every single Ponyville pony wearing clothes does not mean that there is no market there for clothes, as well. Many Ponyville ponies do choose to wear clothes, such as the Granny Smith, Applejack, the Mayor, Rarity's parents (although that can be kind of expected), postal pony uniforms, the vet pony's cute lab coat, the ENTIRE TOWN during Winter Wrap up, not to mention all the costumes the ponies periodically make with their parades and festivals and so on. With the fact that we have never seen another Ponyville clothes store in mind, it could even be the case that Rarity has cornered a niche market in the Ponyville clothing sector, and is rolling in dough. There is at least enough evidence supporting this as there is evidence supporting your outlandish theory that promotes a sex industry in Equestria which has never been shown or referenced, even in passing.
Although it's true that she's only recently begun to be known outside of Ponyville, how long she's been working as a designer is not known, similarly to how the ages of the ponies are also unknown. She could have just been starting out, as she is certainly by now a rising star in the Equestria fashion scene, having won an award, had her dresses featured and modeled by prominent celebrities, and formed quite a number of connections in Sweet and Elite(or at least Fancypants).