One reason often given as to why prostitution is immoral is that it tends to involve coercion. In most cases, people do not freely decide to become prostitutes. In some cases, they are driven to the profession by desperation and a lack of other opportunities for employment. In other cases, they are forced into prostitution by others. In some cases, people are enslaved and forced to be prostitutes.
Such coercion is clearly immoral, especially the sort that involves slavery.
However, some people freely and knowingly chose to be prostitutes. In these cases, the coercion argument obviously fails.
It might be argued that no one would freely chose to be a prostitute and that all people are coerced into doing so. For example, feminists often refer to the coercive power of the patriarchy that is so powerful and subtle that women often do not even know they are being coerced. If these feminists are right, then all (or almost all) prostitution in a patriarchal society would be immoral.
Of course, if we accept this sort of view, then it would entail that almost all jobs are immoral. After all, everyone who is not the top of the power and economic hierarchy will be coerced into working by those above them and by the very nature of capitalism.
The second main moral concern about prostitution is that it is exploitive. As presented stereotypically in movies, prostitutes typically work for a pimp or a madam who takes a sizable cut of their income. This is exploitive because the prostitute is doing the hard work while the pimp/madam is taking an unfair share of the proceeds.
It can be replied that such exploitation is acceptable when it is withing a certain degree. So, for example, the exploitation of the prostitutes by their pimps is exploitive because he takes far too much.
Now, if a degree of exploitation is acceptable, then prostitution that involves exploitation in this range would be acceptable. For example, working for a generous pimp or madam would be a morally acceptable job, on par with working for Starbucks. Once again, capitalism and prostitution can be bedfellows (and so often are).
Of course, if all exploitation is wrong, then almost all jobs would be immoral.
A third reason that prostitution is regarded as immoral is that it is supposed to be degrading to the prostitute.