For centuries, marriage was a straightforward transaction: the woman provided sexual and domestic (to a greater or lesser degree depending on social status) servicing and bore children, the man provided her with food, shelter, clothing, protection and status. Some marriages still run pretty much along these lines, some of them happily, some of them not. Sex work (outside of those cases where there is coercion) is the same transaction only at a much hourly rate, with more free time.
This is, of course, one of the reasons sex work is so stigmatized, it's seen by some as 'spoiling the game' if women are 'permitted' to negotiate the rate, see however many clients they choose, take the money and not have to do the household shitwork as well.
And there are other jobs which are psychologically demanding and distressing, or physically very dangerous. Some of these are percieved as noble (which may help with the mental stress, or not) some are also stigmatized (think debt collector for one of those 1000% APR doorstep lenders, for instance). Many of the most mentally and physically exhausting jobs are paid fuck all and involve a lot of physical contact with people one may find unappealing (think care worker, cleaning up human shit on a regular basis for just about the minimum wage).
I do think that the stigmatizing of all sex work does serious harm to sex workers both willing and coerced, in that as/when/if they move on from sex work, they have to hide their pasts or they are seen as tainted - this is one of the things that keeps some women doing it when they have really had enough, the fact that they can't afford to give up the high level of income they are getting because they can't get a well-paid job.