it is a yes and no question. Technically speaking, the sex industry is immoral. But law should not be set base on morality, as it is not our place to tell other people whether they should be moral or not. The principle of good and evil, is base on whether the action of one person cause disturbance towards peace and harmony between people and society. If it cause no disturbance towards peace and harmony, u do no wrong, if it cause disturbance to peace and harmony, it could be constitute as inappropriate, but based on the circumstances. For example, breaking into people's home and stealing is inappropriate, but if it is a spy, stealing classified enemy document, that is a different story.
Technically speaking, despite it is disgusting and immoral, we can't denier sex industry itself cause no disturbance towards social peace and harmony, so to that extend it should not be illegal.
However, when we look at things in more detail, we realized it is more complex. For example, prostitute frequently easily transmit sexually transmitted disease and too frequently, women working in the adult industry are in fact sex slaves, for example, in the US, pimps, are the one making money, where gangs use threats to force a girl to go into the sex industry, because, there are few women, who is interest in been a hooker and the girls frequently is afraid to go to the police, because what they are doing is illegal.
What we need to do, is have the sex industry as a heavily regulated industry. This include firstly, all women who practice prostitution, must have a license to do so. And all pimps and brothels must also have the necessary license. The license need to be renew on a yearly basis.
The license is to ensure, the girls are free from sexually transmitted disease and is doing it out of her own free will and is not been taken advantage of and the brothel and the pimps only use girls who are doing this out of their own free will and is not been taken advantage of.
If a brothel or pimp is found using a girl that does not have a valid license, the brothel owner or the pimp can go to prison. A girl who practice prostitution without a valid license, will go to prison, unless she is working under a brothel or pimp (in case she might be forced). I think this is the best policy regarding the sex industry.