Fundamentally, I don't have a problem with a woman (or a man) privately exchanging sex for money, if they are genuinely willing. None of my business, as it were. Although, on a personal level, it assaults my feminist principles that people would be having, basically, unwanted sex, because they need the money.
However, I do have a problem with the gendered nature of prostitution - and it's impact on sexual equality of women.
I also have a problem with the legality and social acceptability of men (and it is mainly men) buying people (mainly women, and worse, girls) for sex. It makes women a valuable commodity. And then you get into the trafficking. That's my problem.
Anything that 'can' be sold, will be. Women are bought and sold in organised sex-trafficking criminal gangs, and this hugely escalates in places of lawlessness (eg, the former Yugoslavia, just after the war) or where it is legalised (eg Germany) where it can be passed off as voluntary.
Watch the Whistleblower on Netflix.