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.