The fact that there will always be prostitutes is not a solid argument supporting the legalization of prostitution--there will always be nearly every type of crime and vice.
As for the situation abroad, I think it varies greatly. In China and Africa for example, most prostitution seems somewhat voluntary. The prostitutes are often poor and need money and don’t see alternatives, but they are not physically forced into brothels. In contrast, in India, Nepal, Cambodia, Laos and Malaysia, the majority of prostitutes are initially forced into prostitution. The slavery is real. Then after some months they are given greater freedom, and maybe at that point they are held in debt bondage but not physically restrained. And some is clearly voluntary: When girls are liberated from brothels and taken to shelters, some run back to the brothels.
Prostitution in China | Prostitution | China
“In any case, the debate about legalization of prostitution seems to me to be a distraction. Whatever one thinks of whether 18-year-olds should be able to sell sex, everyone can agree that 14-year-olds shouldn’t be imprisoned inside brothels. So let’s focus not on prostitution, but on slavery — and make that the priority battle.”