He addressed it directly in 1844 in "Private Property and Communism" Rather than understand prostitutes as "owning the means of production" he viewed them as laborers, in light of the fact that there were those who were prostituting them. Here's the direct quote:

Prostitution is only a specific expression of the general prostitution of the labourer, and since it is a relationship in which falls not the prostitute alone, but also the one who prostitutes – and the latter’s abomination is still greater – the capitalist, etc., also comes under this head. – http://www.marxists.org/archive/marx/works/1844/manuscripts/comm.htm

Interestingly, this came in a piece where he was mainly criticizing other forms of communism for how they treated private property. Marx was making the distinction between universal private property and abolishing private property. Perhaps a fine distinction. Prostitution, then, comes into the picture because Marx is discussing the possibility of women becoming universal property an idea which he was vehemently against as it denied their personality/personhood.

To wit:

This type of communism – since it negates the personality of man in every sphere – is but the logical expression of private property, which is this negation. General envy constituting itself as a power is the disguise in which greed re-establishes itself and satisfies itself, only in another way.

(see above link for source)

In continuing the talk on his ideas on prosititutes being workers, he also viewed quite negatively the Lumpenproletariat, or "refuse of all classes." From The Eighteenth Brumaire of Louis Napoleon (1852):

Alongside decayed roués with dubious means of subsistence and of dubious origin, alongside ruined and adventurous offshoots of the bourgeoisie, were vagabonds, discharged soldiers, discharged jailbirds, escaped galley slaves, swindlers, mountebanks, lazzaroni, pickpockets, tricksters, gamblers, maquereaux [pimps], brothel keepers, porters, literati, organ grinders, ragpickers, knife grinders, tinkers, beggars — in short, the whole indefinite, disintegrated mass, thrown hither and thither, which the French call la bohème.