Prostitution actively causes gender inequality and degradation. The existence of prostitutes reinforces cultural attitudes of women being sexually subservient to men and therefore has negative implications for all women.
As Hassan states, “If prostitution is legal, and men can buy women’s bodies with impunity, it’s the extreme sexualization of women... if women are sex toys you can buy, think about the impact on relationships between men and women, in marriage or otherwise.”
The customer is always right, and the sex worker is considered by those paying for sex to be nothing but an object existing for the purposes of fulfilling their desires. Prostitution promotes the idea that male sexuality must be satisfied, without consideration for the wellbeing of those who satisfy it.
Prostitution is often portrayed as being needed due to insatiable male sexuality,
or even as a way to curb men committing sexual violence.
The idea that male sexuality cannot be controlled is deeply harmful to women, and insinuates that women have a sexual obligation to men.
The sale of sex by sex workers fundamentally rests on a dynamic of inequality.
No woman wants to become a sex worker; it is done purely for the money which is used as an incentive to buy into a system of inequality and degradation. This is a system in which the people profiting off of sex workers, rather than the sex workers themselves, benefit. Functionally, prostitution is simply the mass profit of sexual exploitation by ‘pimps’. 
This dynamic of inequality means that the very concept of choice within prostitution is non-existent. The theory of substantive autonomy posits that only certain choices are valid - true 'choice' does not exist in situations where the thing being chosen is inherently oppressive.
For instance, it would not be a valid choice for someone to choose to enter slavery. In the same way, as prostitution is an inherently oppressive act, it is impossible for those involved to be said to be making a truly autonomous choice.