"Lumpenproletariat" is exactly the kind of idea an educated German theorist would come up with in the wreckage of the industrial revolution and it's ridiculous to try to carry that notion forward to the age of cell phones and heavily armed maoist prostitutes and if anyone can't understand that you should throw grass at them until they stop being dorks because they're too far gone to touch it themselves.
Like ffs read even one anthro text about black market and grey market economies and stop treating The Man's legal system like anything but a criminal organization.
Genuinely though, it's hard for me to imagine even a petit bourgeois Marx thinking that sex workers are literally dangerous.
Just based off that I tend to think that he didn't mean physically dangerous but socially dangerous. I just don't get it otherwise.
People's brain worms about sex work runs so deep it often seems like the foundation on which they've built everything else. Some of it can be explained as physical danger: sex workers were viewed with some justification as the primary vector of transmission for stis. Back in the day that was a very serious public health problem as illnesses that are treatable today caused enormous harm back then. But that's a public health concern. It has an economic dimension, but the actual labor of sex workers cannot be set apart from that of anyone else who works with their body on the criteria that they were often sti vectors.
There is a social concern that sex workers "break up families", and if that is taken seriously it privileges christian moralism and the boug's legally enforced nuclear family, as some kind of actually existing economic force instead of a flimsy legal fiction that acts as a means to discipline and control workers and enforce social reproduction of labor.
In the final analysis the loathing of sex workers, and their dissmissal as passively victimized minors (in the legal sense of a person who is not legally considered to have agency and responsibility) with no awareness, no agency, and no potential, who can only be patronizingly destroyed for their own good by their moral and intellectual betters, reflects a rank and basal hatred of women and the sexuality which is inherent to them.