As a society we need to acknowledge that the real reason prostitution is illegal is because historically - and in many cases even today - sex is viewed as inappropriate, impure and undesirable apart from true affection. Some fewer people also consider commitment to be part of the necessary context for sex; and others, a lifelong monogamous two-gender commitment blessed in a religious context.
Even in the context of marriage, should it cease to lack affection, many people disapprove of sex - but by far the most disapproval is reserved for two instances of sex outside of mutual affection - rape and prostitution.
In one case, raw force is the factor that permits sex in an inappropriate instance; in the other case, there is some sort of material exchange that allows for consent, when one might expect consent to not otherwise be forthcoming. Whether the woman puts herself up for sale, out of ambition or desperation, or is put up for sale by a third party seeking to profit from her, the act is still sex devoid of affection between the partners and as a society, we generally condemn it.
This condemnation need not be associated with any external acts of abuse or crimes outside of the sex itself - and to try to justify it that way is merely a nod to the few people who seem to believe that sex is merely a sort of pornography where the pictures come to life; that both people imagine in their own heads the interest or participation of a partner, and please themselves as best possible, while perhaps doing what is necessary to ensure a repeat performance at a later date.
For people who experience sex that way, we should perhaps have pity. But there is no need to bow to them in setting public policy.
