I think the main issue with legal prostitution is finding a way to regulate that keeps it safe for all concerned and keeps the pimps and organized crime out .
I do not think it should be illegal to sell sex. I do think that brothels/massage parlors or home-based businesses are greatly preferable to street walking but I can understand the desire to not have pimps/madams profiting off this. The last thing we need is for this to go corporate and have Wall Street pressuring pimps to keep those sessions short and wages low. I think condoms and regular STD testing should be mandatory (a lot of the porn biz already does this).
The problem is you can structure the biz relationship many ways to avoid admitting the reality that a paid security guard is often going to actually be a pimp or that a booking service is really going to be a madam or that a motel or red light rowhouse is really just a brothel with a brothel owner making most of the money. How do you make sure that the vast majority of the money stays with the workers and that they don't end up being exploited by the owners of red-light rooms, or "security guards".
That is one reason why efforts to ban ads on craigslist and elsewhere on the internet are counter-productive since craigslist provides a safer way for sole-proprietors to find clients without relying on madams or walking the streets and then needing "security"
In terms of harm reduction and simple good policy, legalizing both the activities of the prostitutes and their customers would go a long way towards keeping things safer.