It's the idea that "lady-boys from Thailand" are the only ones sex trafficked that allows people to believe they aren't part of the global blind-eye we turn to what the UN calls "modern day slavery". In areas of legalized prostitution (like Vegas) it's proven there's an increase in human trafficking. (search pub med and google, multiple studies) Because most everyone believes that human trafficking happens in southeast Asia or some 3rd world country, they get away with it in the US too. People don't pay attention to what they see. I'm not saying there aren't some people who choose prostitution, but most are trafficked.
In the definition of trafficking, the Department of Health and Human Services includes anyone under the age of 18 (automatically sex trafficked) and anyone with a pimp. A pimp being one who controls the "sex solicitor" and abuses them if they don't make enough money. About half of sex trafficking victims are US citizens, as well as others brought in from SE Asia (15,000-50,000 trafficking victims are brought INTO the US each year, adding to the number already here being trafficked).
Human trafficking statistics from the Polaris Project:
http://wiki.preventconnect.org/file/view/Human+Trafficking+Statistics+from+the+Polaris+Project.pdf
I've talked to a doc who said at 3am the South American women came to see him in the ER in DC for their STD checks and trid to get in and out as fast as possible. He knew they were trafficked, but there was no system in place for him to do anything to help them. They'd just get beaten by their traffickers (who were often in the room with them) if he tried. He just did his job.
Physicians, because of what we do, have a greater opportunity than most to do something. "Doctors Not Trained to Spot Sex Trafficking Victims":
http://www.reuters.com/article/us-sex-trafficking-recognition-idUSKBN0MC1XE20150316
If you are aware and you look, you'll see it. Educate yourself.