Have you tried just being a prostitute a bunch? You get it when you have "lady of the night" at rank 20 (I think that's what it's called, the skill you rank up each time you work as a bottom in the brothel).
You can get the class as early as Lady of the Night 10 by failing a skill check, but once you hit Lady of the Night 20, leave the brothel and return. You should enter dialogue with the madam and get the prostitute class
