Prostitution is known as one of the world's oldest professions. However, prostitution is illegal is almost every city in the United States. Therefore, black market prostitution is often used in inner cities, truck stops, and even in suburban areas. By allowing prostitution to be legitimized, it could be better regulated through policies such as STD testing and the process of monetary sex could be a much safer one.
Because the sex industry involves so much risk, there's a myth that it's most responsible for the spread of STDs. Therefore (so the line of thinking goes), if we got rid of it, STDS would magically disappear. However, if you look at the stats, you'll see that STDs is mostly widespread amongst various demographics that engage in unsafe sex and yet have nothing to do with the sex industry (people who don't use protection, randomly hook up with strangers, etc). Legalizing prostitution is nice and all, but it would hardly put a dent in the spread of STDs as long as you have these other groups of people engaging in high risk activity.