To start, a comment about human nature.
Many liberal feminists push back on any claim that intrinsic psychological differences exist between males and females. The assumption is that our brains come to us as empty "blank slates" that are wholly shaped by culture. Consequently, any differences we observe between men and women have to be due to environment, culture, and childrearing.
Now, it is true that we observe statistical variability among men and women. Each person needs to be treated as an individual. But when we step back and look at population-level statistics, we do observe differences between the sexes. Many of these differences are slight, and often get overplayed as "differences." But some of the clearest psychological contrasts we observe between men and women concern sexuality. (The other area of big contrast is aggression.) And the best data point we have about gender differences in sexuality involves the oldest profession.
Simply put, men seek out casual sex to a scale we just don't observe among women. This creates a supply and demand problem, with more men seeking sex than there are women willing to have sex. Throughout human history this sexual asymmetry was handled through prostitution, with the burden falling largely upon women in vulnerable economic situations. From time out of mind, poor women have been forced to satisfy the demands of male sexual desire.
This is the basic injustice at the heart of the oldest profession, the economic oppression. By and large, liberal feminists ignore this fact, believing that the gender asymmetries we observe between sexual buyers and sellers in simply the result of stigma. But stigma has nothing to do with the fact that the overwhelming majority of women forced into prostitution do so as a grim act of survival rather than as a path toward self-actualization. Perry writes:
[Male desire for causal sex and sexual variety] produces a mismatch between male and female desire at the population level. There are a lot more straight men than there are straight women looking for casual sex...As we have seen, in the post-sexual revolution era, the solution to this mismatch has often been to encourage women (ideally young, attractive ones) to overcome their reticence and have sex 'like a man', imitating male sexuality en masse. The thesis of this book is that this solution has been falsely presented as a form of sexual liberation for women, when in fact it is nothing of the sort, since it serves male, not female interests...
Our modern solution is to encourage all women, from every class, to meet the male demand for casual sex. In contrast, the solution adopted by most societies in the period before the invention of reliable contraception was for the majority of women to have sex only within marriage (whether that be monogamous or polygynous), while a minority of poor women were tasked with absorbing all that excess male sexual desire. Aside from a handful of high-class courtesans and call girls who might attain some degree of social status--usually having come from poor backgrounds originally--the prostituted class has historically been composed of women with no other options: the destitute, those abandoned by their partners, those addicted to drugs or alcohol, and those captured in warfare or tricked by traffickers. Prostitution is an ancient solution to the sexuality gap, and it is not a pleasant one.
It's very difficult to explain the wretchedness of the prostituted class if you believe the modern liberal feminist claims about the sex industry...
The whole point of paid sex is that it must be paid for. It is not mutually desired by both parties--one party is there unwillingly, in exchange for money, or sometimes other goods such as drugs, food or shelter. The person being paid must ignore her own lack of sexual desire, or even her bone-deep revulsion. She must suppress her most self-protective instincts in the service of another person's sexual pleasure. This is why the sex industry typically attracts only the poorest and the most desperate women--these are the people who don't have the means to resist it.
I think it is deeply compassionate to want to reduce the stigma associated with sex work. In this, we follow the example of Jesus himself. But liberal feminism is confused about the proper focus of our empathy. The point isn't to reduce the stigma because it is sex work, that our problem with prostitution is a prudish and puritanical attitude toward sex. Sex isn't the issue here. The problem isn't stigma. The problem is economic oppression.
