Prostitution can be seen as exploitation even if the women aren't overtly forced into it by pimps or human traffickers, because many of them are in the business because of poverty, drug addiction, or past sexual abuse.
The article I posted references a survey of prostitutes in the Netherlands that showed 79% of the women wanted to get out of the business. It's obviously not good for your emotional health to have meaningless sex with thousands of people you'd never touch if they weren't paying you. Most prostitutes would probably prefer a typical monogamous relationship, and it's a lot harder to have that when you're in a business that requires you to routinely practice unhealthy behavior toward the opposite sex. When you're only consenting to sex because you're economically coerced by your job, it's not genuine consent.
Another quote I came across:
"It is impossible to use a human body in the way women's bodies are used in prostitution and to have a whole human being at the end of it, or in the middle of it. And no woman gets whole again afterward."