There are a lot of reasons.
Prostitution puts underage girls at risk.
Prostitution exists largely as economically coercive. The vast, vast majority (95%+ are only in sex work because they have no over options.)
Legal prostitution increases violence against prostitutes, and sex trafficking.
It's inherently oppressive to women, and is used to cover other illegal activities, whether legal or illegal.
There is a childhood sexual abuse/rape ----> sex worker correlation that is incredibly strong. The "damaged girls" myth is entirely true, and sex work covers it up.
PTSD can be a result, and prostitution blurs the lines between consent and nonconsent, i.e. Rape.