I see pros and cons to sex worker unionization. Let's say prostitution were still criminal but there were more unions, to set wages, etc. The unions may be great for girls to find a support network, information sharing, etc., though many girls already find this online.

On the other hand, I totally agree with Ren, that competition lowers prices. Unions, in my understanding, exist for two reasons: one, to place workers in a bargaining position with management, and two, to limit the forces of competition to keep wages from being undercut. I could see this working in a strip club, a brothel or an agency environment, as the sex workers could bargain with club owners/madames/management in general for certain work conditions, hours, minimum pay, etc.

But free agent/independent prostitutes (i.e. non coerced, those doing it of their own free will) already choose their own working conditions and hours. If unionized, with whom are they going to bargain for pay? The Johns? Haggling is already de rigeur between hobbyists and providers. And are they going to bargain with each other? I doubt that there's going to be widespread acceptance of an idea that all girls must work for a certain rate. Some girls are happy offering full GFE for a full hour for $100. However, plenty of girls can command $500, $800, or more an hour. The $100-an-hour girl, though undercutting prices, isn't necessarily hurting her more high-priced competitors, as they're not in direct competition to start with. The $100 girl is more likely to be less attractive, offer a "down and dirty" session, and be less picky about her clientele, sort of the "working man's" provider. The $800 girl is more likely to be a porn star or businesswoman type that a man may want to take out in public to a function. Sorry if that sounds mean or stereotypical, just what I've witnessed in my experience. 

I don't really see where unions would help, or even come in handy, for independent providers, as there would always be scabs willing to work for less, and men looking for a bargain.