It appears most women stay away from this thread... Oh, well.
There is very little reliable statistics on trafficking. This article, even though it talks about sex trade in Thailand, address the universal challenges of finding data - despite a huge number of human rights organizations that feed on it.
http://www.prospect-magazine.co.uk/article_details.php?id=6889
I cannot find links right now but a friend of mine, a decriminalization activist in the US, showed me some papers claiming that often, we deal not with women coerced into prostitution completely against their will - but women who expected to work under different, much better conditions.
Either way, even if a woman from an impoverished country knew she was agreeing to sex trade - if she is forced to see more people, do acts she hates and gets far less money for it than she expected, it's still pretty bad.
As Anita said (I think), you are more likely to find women like this in micros/Asian MPs than anywhere else. In other words, the cheapest places. And even if a woman is there voluntarily, she is still most likely supporting her extended family back home on that money.
Let me give you an example from a different area. I like organic products. I like the principle behind them. I am not a vegetarian and am not planning on becoming one - but I believe in treating animals well. As a student, I could not afford organic or free-range products. But after graduation, I made space in my budget for them - even though I had other uses for that money. I liked their principles so I chose to support them at a personal cost, sacrificing going out or buying nicer things.
Yet some of the men out there do not make a similar choice. They want to haggle over $20 - which could buy them a couple of beers - or support a family for several days in Asia.
I have to make a disclaimer here! I believe this argument mostly applies to micros/ethnic MPs, i.e. places where the women are most likely to be immigrants from a third world country with a lot of family left back home. If you are visiting a Canadian lady, she is facing same prices as you are, so $20 probably has an equal value to both of you. In other words, I am not against seeking a good deal - only when the deal comes at a significantly higher cost to one party than the other.
Yes, you cannot tell whether the woman is there by choice, or under what conditions she made that choice. However, you can make her life significantly better at a comparatively lower cost to you. Yet many choose not to. I don't care whether it's labelled rape, exploitation, or smart pooning. It's still a choice I view as unethical.