I think what might be at stake is not just morality but capitalist profits as well. If the capitalists were to pay in full for the reproduction of labour power - which under the current arrangement is defined as housework and either unpais or paid for by the general public (c.f. education) - that would be a serious blow to profits, as the example of the x-USSR demonstrates.
So keeping the reproduction of labour power outside the market relations as much as possible reduces the exchange value of labour power to the 'socially necessary' miniumum - and that follows directly from Marx's theory of value. In other words, the separation of the 'domestic' and the 'professional', housework and employment, is fundamental to capitalist profitability.
Sex work violates that principle of separation and that, IMHO, is the main reason why the guardians of the capitalist order do everything to degrade it, even if individual capitalists venerate it is an epitome of 'entrepreneurship'.
It is, BTW, another contradiction of capitalism: it represents the pressure to commodify even those area whose commodification will eventually erode capitalist profitability. It is why corporate schmucks, from Clintonoids to the World Bank types and financial speculators like Soros now promote 'civil society' - meaning: voluntary service sector.
Regards,
Wojtek