(“Taking a position on sex workers” is an awkward turn of phrase. So I’ll instead give an opinion on natural law of prostitution.)
1 – As far as I know, since males must work for female services, and since, females (largely) decide, all male-female relationships consist of some sort of exchange. So, all but causal male and female relations are simply on a spectrum of prostitution – from purely reciprocal affection through to purely commercial service.
2 – Prostitution (anywhere on the spectrum) can consist of a voluntary, productive, fully informed, warrantied, exchange, free of imposition of costs upon others by externality and is therefore ethical (interpersonally) and moral (extra-personally).
3 – Practiced as a means of obtaining additional income in order to pay down debts, or to create investments, (the better Call Girls and Escorts) seems is very hard to argue with – although this tends to permit prostitution as a luxury for the middle and upper classes when it’s the working and lower classes (and especially the underclasses) that are both most likely to engage in it, and who are most likely to abuse it (for affection, for income), and then require medication (drugs) to tolerate it.
4 – It is traditionally a last-ditch means of income for those who cannot find equal income by more desirable means. In many cases it is a means of economic survival. It is almost never a career a woman wants to remain in. It is just more economically rewarding (and more independent) than her available alternatives. And as such it is a difficult and unpleasant career to exit. As such it causes a dependency spiral.
5 – The principle problem with prostitution, like many, many moral questions, is the externalities, not the activity. (a) it creates the greatest and most expensive moral hazard of unwanted reproduction, thereby burdening all of society, and (b) it is not an occupation that we would wish our family members to engage in, so we are constantly defending against it (just like credit, drug, and alcohol use). (c) it is a constant threat to breaking up families given the unnatural stress of intergenerational monogamy during the first twenty years of marriage when women devote attention to children because they must, and a man most feels the ‘demand’ for intercourse. (d) And families serve as the first industry, the first corporation, in the hierarchy of corporations we call ‘civilization’. (e) And while in history, prostitutes and traveling workers and soldiers fulfilled one another’s needs, by the 20th century we had achieved nearly universal pairing off (marriage) along with the near abandonment of migratory craftsmen and migratory soldiers.
6 – Ergo, the principle issue with prostitution is – like all things – keeping it invisible to the commons. So like all sex, it is not a problem in a bedroom, or between individuals. And it is hard to argue with it as a ‘sugar daddy’ or ‘mistress’ or ‘select clients’, or ‘side job’. It is a problem, when it is either necessary for survival or transforms into a condition that requires medication. Since that medication is evidence of spiraling. And spiraling always ends up externalizing the costs upon the rest of the polity.
7 – No person may take an action the restitution for which he cannot pay. And one simply cannot pay for the consequences to society that are produced by destruction of marriages, loss of life’s potential, and the vicious spiral of decline, medication, and crime that results from it.
8 – Therefore, liability for spreading disease, liability for fomenting divorce, liability for encouraging others to engage in prositution, liability for supplying medication to preserve someone’s participation in prostitution, liability for creating the moral hazard of prostitution (baiting, entrapping, trafficing), and liability for ‘polluting the commons’ are all liabilities one must avoid – because one cannot perform sufficient restitution to correct for those consequences. But if any person out of public sight and sound offers and negotiates an exchange of sex for money – that is entirely ethical and moral.