"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.
Under a communist society, social labour is socially planned.
Possibly, there will not be any allocation of labour into sex work. But if 2 consenting adults exchange favours for sex, then there is also no reason for the government to get involved.
Another possibility is that the government does allocate labour to brothels, but this is a political question of priorities whose answer depends entirely on the citizens of this hypothetical state.
There are some arguments as to why any communist state that is created today shouldn't allocate labour into state-run brothels. Any existing communist state would face massive challenges that it should prioritise.