If only it where so simple. this is quite a complex issue. This may get a little long, as it will touch on a lot of things.
First of all, I am totally ok with the idealistic version of prostitution, and im am totally ok with the idealistic version of drug use. but in reality, it does not quite work.
A very large amount of prostitutes are indeed doing it to feed a habbit, is that a problem? well, it would not be if their habbit where legal. its their choice, if they could go to a drug store and buy their drugs, it would be fine. however, drug money does fund criminal gangs, so does some forms of prostitution. this is a fact that has been verified time and time again.
Prostitution also does spread disease, specially when combined with desperation to feed a habbit. where prostitutes will say no condom is ok to try and get the client.
But, the main problem. is slavery. there have been reports that say as much as 27% of prostitutes are forced in to the job, even in areas where it is heavily regulated, such as amsterdam. in other words, having sex with more than 4 prostitutes can almost be considered rape.
Girls are taken from poor areas such as lunik 9 and other areas, and then forced for years to do all kinds of things.
Just because a country is modern does not mean you are free from slavery in the sex trade. say for example the USA, i bet you could find a lot of prostitutes from poor countries that are working in the USA as prostitutes, who are forced in to doing so. cubans, mexicans and so on. here in europe, in spain, they come from poor eastern european places.
should prostitution be legal, i think so. but the regulations should be very heavy. no pimping, no slavery and a load of other rules.
do some prostitutes do it just because it is easy money, and do it out of their free will, yes. lots of them do. millions in fact. but it is also true that millions more are forced in one way or another. and trying to argue in favor of that is basically trying to justify rape.
personally, i think the solution would be to make it legal, as well as make drug use legal, and have a lot of regulations to go with it. but, as places like amsterdam have shown, even like that, there will be some problems.