I think morality plays into it. If morality wasn't an aspect of this, you could say the same thing about rapists: they're universal in human society, they're at

I think morality plays into it. If morality wasn't an aspect of this, you could say the same thing about rapists: they're universal in human society, they're at a unique high risk of abuse and discrimination. But criminal history is not a protected class because society's morality says we shouldn't protect them. This isn't to say that sex workers are like rapists, but only to illustrate that the spectrum of morality influences what is considered a protected class. And despite what people might want it to be, sex work has pretty consistently been in a gray area of morality: both desired and rejected. I don't think it is at the threshold to make it a protected class, at least in the USA