the essential task of capitalism is to make transactions of what used to be activities of daily life. We humans used to spend our time hunting and gathering food, but I can honestly say that I have never hunted or gathered in my life, we have professionals and machines to do all that for us. If capitalism can make a transaction out of anything, it is probably going to make a transaction out of sex.
Virtually all sexual relationships are transactional in some way. The old adage that marriage is a form of prostitution is not so wide of the mark. Perhaps the stigma of sex work comes not from leveraging one's sexual capital in itself, but by setting the bar too low - essentially making oneself available for casual employment as opposed to taking only a full-time job. To a high-priced call girl, a woman willing to sell sex relatively cheaply is potentially a slut.
I suppose if the stigma of sex work were to vanish and all women felt perfectly free to sell sex on demand (and men were perfectly free to purchase it), then this would have some effect on the male demand for marriage, with men perhaps being less inclined to consider that option. For women whose station in life depends on securing the best marriage outcome possible, sex work represents a flooding of the market. I suppose it is only natural that some women would tend to disparage the paid product as cheap and inferior, to to speak, and to try and maintain the stigma attaching to sex work as much as possible.