This thread is a bit of a trainwreck but not really more so than I would have expected.
I think it is pretty clear that the way most women and men in society approach sex is the direct explanation. A very basic thing is that women tend to be attracted to men that have status (in some vague, broad meaning of that word) and male sex workers really don't. While men tend to care less about that when it comes to just choosing someone to have sex with.
Reasons behind that are probably both biological and social to some extent.
One thing that is true in my experience (although this is anecdotal obviously) is that female sex drive is much more dependent on external stimuli (for lack of a better term). That is, many women can go for a long period of time and not particularly want sex and then want it a lot when they find someone they're attracted to. Men tend more in the direction of wanting it to a similar extent regardless of whether anyone else is around. Again, these are all generalizations that obviously don't hold true for every individual but it seems like a logical reason for different prostitution rates.
That is what an economist would call the demand-side of the sex market. If you were to look at the supply side I suspect social reasons become more important for what people think they can do and what can be expected of them.
As an aside, people mocking the idea of male entitlement to sex are being weird. I kind of struggle to imagine how unobservant one must be to not be aware of that. The same is true of female sexual repression. And having grown up in educated, liberal, urban society in Sweden I'm probably in one the most progressive environments that exist.
