TL;DR - for the vast majority of female sex workers in that country, it is an economic CHOICE. The mostly single mothers felt it was their best opportunity to earn an income capable of supporting their families.
Those with agendas (especially those who are after your donations) are always the ones who speak the loudest.
Of course, not every country is the same, not every scenario is the same, and thus we can't say that the choice some German families made after WWII and those single mothers in the Philippines are making the same kind of choice, are they?
Make no mistake - the vast, VAST majority of sex workers in the world are being exploited. Sometimes it's in the form of violence and coercion to continue the job. Sometimes it's due to unfair social and economic pressure on a vulnerable demographic. Sometimes it's just plain-old sexism, misogyny, humiliation, degradation, etc that's taken for granted as part of the career, which some workers have more agency in allowing, while others that have to take whatever abuse comes their way.
For the most part, my friends who are sex workers and enjoy the work are people who have unlimited choice: stable, single, educated, white, middle-upper-class women who could probably get any job they wanted, and choose this career and their clients.
My friends who are sex workers and don't enjoy the work have effectively no choice in the matter, either because they don't have an effective education, they have an inability to get other regular work, are destitute, have no social support structure, or have emotional or psychological disorders. And these are the people who are not being pimped out or sold for slavery.
But I probably don't know what i'm talking about, because I didn't write a master's thesis on it.
So why couldn't they CHOOSE to be CEOs or middle class professionals?
It's a serious question. Even allowing for IQ and aptitude differences, the idea that this is a genuine choice is difficult to support - because it's only a choice in the sense that they're given very limited options in a social situation that's heavily rigged agains them.
Perhaps it's also relevant to ask:
1. How many would do it given a completely free choice without external economic pressures they have no control over?
2. Why do they not have a completely free choice?
That does raise a lot of interesting questions. How did they get into the trade ? - This is usually where the primary exploitation happens. After that, it is not uncommon for women to stay within the industry by choice, (because they don't know how to do anything else, they have debts to pay, shame/stigma and trouble reintegrating with society and family after being outcast). Who are the fathers of the children - Are they from an actual relationship or are they consequences of the trade? Of course, all my statements are primarily from media coverage or anecdotal in nature. I don't know how things are in the Phillipines. Could you share a link to the paper? Might be a good counter-point on the matter which people can cite.
edit : typos
But I'm pretty sure that you'd have slight reservations if your wife or daughter chose this kind of job. Why? It'd make perfect economic sense.