Indeed, I agree with you that part of the argument that prostitution is legalized in some countries may reduce risk.
But first, prostitution can not be legalized in some other countries in the world, so the prerequisites you have said are no longer right. Second, if the legalization of prostitution, will lead people into the industry, causing more risk of disease transmission. Finally, even if the prostitute on a regular physical examination, when she knew that after a lot of people may have been infected with the disease, because the industry is large, spread a wide range, will cause great losses. The spread of a new disease in the extreme time is not easy to find a cure method, which is the risk of legalization of prostitution.
Finally, the legalization of prostitution can lead to a lot of kidnapping, trafficking and the like, many people in order to profiteering or other causes of crime, society will become increasingly chaotic.