"Only" in the U.S. is sexual work so taboo.
As someone who's travelled all over the world, in a lot of countries sex work is legal. Not just third world countries, but Europe, UK, etc.
In my 20s I was against sex work for the most part as I still enjoyed the thrill of the chase, now in my early 30s I understand. When you have a demanding career, wife at home, kids, etc. You don't have the time nor would you want to risk anything getting back to your family. You just want to have sex and keep it moving and there is a whole industry of woman willing to do this.
Like Kev said, until you're exposed to that kind lifestyle you don't really believe it.
All that said, Kev is referring to high end escorts. I know 3 girls myself that are in that high end escort life and they've told me that with some guys they don't even have sex with them. The men just want the Girl Friend Experience (GFE)