Historically, women had VERY limited options to provide for themselves if they became widowed, outlived their families or fled their childhood homes ~ and in many places on this planet (to add a 'travel' angle
) the circumstances for women are similar to the historical roots of prostitution. There was a market for their services then, all through history and there still is; and just like any other commodity if you remove the demand you diminish the stakeholders.
Choose to be a prostitute? I don't think a person chooses it, but it's a pretty easy occupation to get into if your options are limited.
Relationship deal breaker? Prostitute - probably not, because of the difference between sex and love. I've felt the biological urge and nothing else (I'm widowed and not in a relationship); ONS - yup, shows a supreme lack of self-control and violates trust; Mistress - start dividing your assets, hunny...
Canadian, if that matters.