As the following passages from _P&P_ make abundantly clear, given the descriptions of "sex worker" in many lbo-posts of the last couple days, Charlotte clearly classifies as a voluntary prostitute.
******** In as short a time as Mr. Collins's long speeches would allow, every thing was settled between them to the satisfaction of both; and as they entered the house, he earnestly entreated her to name the day that was to make him the happiest of men; and though such a solicitation must be waved for the present, the lady felt no inclination to trifle with his happiness. The stupidity with which he was favoured by nature must guard his courtship from any charm that could make a woman wish for its continuance; and Miss Lucas, who accepted him solely from the pure and disinterested desire of an establishment, cared not how soon that establishment were gained.
----
She had gained her point, and had time to consider of it. Her reflections were in general satisfactory. Mr. Collins to be sure was neither sensible nor agreeable; his society was irksome, and his attachment to her must be imaginary. But still, he would be her husband. -- Without thinking highly either of men or of matrimony, marriage had always been her object; it was the only honourable provision for well-educated young women of small fortune, and however uncertain of giving happiness, must be their pleasantest preservative from want. This preservative she had now obtained; and at the age of twenty-seven, without having ever been handsome, she felt all the good luck of it. The least agreeable circumstance in the business was the surprise it must occasion to Elizabeth Bennet, whose friendship she valued beyond that of any other person. Elizabeth would wonder, and probably would blame her; and though her resolution was not to be shaken, her feelings must be hurt by such disapprobation.
----
``Why should you be surprised, my dear Eliza? -- Do you think it incredible that Mr. Collins should be able to procure any woman's good opinion, because he was not so happy as to succeed with you?''
But Elizabeth had now recollected herself, and making a strong effort for it, was able to assure her with tolerable firmness that the prospect of their relationship was highly grateful to her, and that she wished her all imaginable happiness.
``I see what you are feeling,'' replied Charlotte, -- ``you must be surprised, very much surprised, -- so lately as Mr. Collins was wishing to marry you. But when you have had time to think it all over, I hope you will be satisfied with what I have done. I am not romantic, you know. I never was. I ask only a comfortable home; and considering Mr. Collins's character, connections, and situation in life, I am convinced that my chance of happiness with him is as fair as most people can boast on entering the marriage state.''
----
It was a long time before she [Elizabeth] became at all reconciled to the idea of so unsuitable a match. The strangeness of Mr. Collins's making two offers of marriage within three days, was nothing in comparison of his being now accepted. She had always felt that Charlotte's opinion of matrimony was not exactly like her own, but she could not have supposed it possible that, when called into action, she would have sacrificed every better feeling to worldly advantage. Charlotte the wife of Mr. Collins, was a most humiliating picture! -- And to the pang of a friend disgracing herself and sunk in her esteem, was added the distressing conviction that it was impossible for that friend to be tolerably happy in the lot she had chosen.
-----
The rest of his letter is only about his dear Charlotte's situation, and his expectation of a young olive-branch.*****
I include the last to underline the "most humiliating picture" which Austen intended Elizabeth's thoughts to convey -- the humiliation of sexual intercourse with so repugnant a person as the Rev. Collins. One suspects that only a minority of a prostitute's clients would be as repellant.
Carrol
P.S. To tie this in with another thread of a year or two ago, the more one reads Austen the clearer it becomes that a core theme in her work is the centrality to human happiness of mutual dependance, and the venom with which she regards threats or blockages to such mutual dependence.