Here's my take:
I live in Colorado and quite frankly, I don't think making prostitution legal will have much of an effect on anything. Sites like Seeking and Sugar Daddy Meet are already very popular and many girls in Colorado use them. Most of the girls are college students who need money to pay for school and rent. Sure, there are a few girls loaded with tattoos, piercings, bright pink hair, etc., but not many.
So what you will likely see here is an influx of girls using those sites knowing that they (and their John) no longer have to risk getting busted by the cops.
I liken it to online sports betting and gambling, which is also legal here in Colorado. Before it went legal, if you wanted to place a bet, you could use Bovada.com (back then Bodog) or Pinnacle.com. I suppose some may have also used old-fashioned bookies, but not many. The sites back then were easily used and they paid out just fine. I used Bovada for years and their customer service was stellar. But now that it's legalized, more people do it on the same platform -- online -- knowing the legal risks are gone.
Colorado will not turn into the wild west with brothels on every corner run by Miss Kitty (Google her) with drunks running wild and shootouts at high noon on Blake Street.
Ciao,
~Dash