Demeaning of sex workers plays two functions: (1) glorification of paid employment, of _any_ work other than sex work; and (2) letting marriage off the hook.
Moralism and sentimentalism of the kind Wojtek speaks of regarding our perception of sex makes us think that while receiving pay for sex is unnatural and to be condemned, receiving pay for any other line of work is natural and commendable. Special moral condemnation heaped on sex workers has an effect of making wage labor other than sex work seem moral.
Also, marriage between partners of unequal economic standings is little different from prostitution, except that in marriage, women are expected to not only perform sex work but also house work, care-giving work, etc....all in the name of 'love.' Demeaning of sex work has an effect of romanticizing marriage.