The short answer is that almost all sex workers are women and 99.9% of all sex workers are working for men. The longer answer is:
1) Male sex workers often have a very different experience from female sex workers.
I remember having a guy friend who worked in-calls constantly telling me that he loved his work and asserting that any women who complained of feeling pressure from male clients were not properly dealing with their clients and that, in fact, these women were bad sex workers. I think that what this man failed to see is that he, a highly educated white man who worked as a high-ranking executive during the day, was going to be treated very differently by his clients than, say, someone like me.
This is not to say that male sex workers do not get abused. They do. But female sex workers are also abused and at astronomical rates, in a system that has disadvantaged them and sexualized them from the day they were born.
2) Patriarchy is a motherfucker. Women are universally sexualized, de-humanized and toiletized and men in our society are taught to be aggressive and to see aggression as sexual. It is deemed socially desirable, even mandated, for men to seek out passive and controllable partners, and to engage in non-emotional, semi-abusive relationships with those partners. When no such partners are readily available and rape strikes him as unpalatable, there is always the option of paying to rape a stranger, thus legitimizing the abuse.
Men who enjoy sex with men might, on the surface, be seen as subverting the patriarchy by seeking out same-sex partners, but their learned modes of sexual expression are patriarchy-approved and often cause them to abuse men in the same way, and sex workers where applicable.
Yes, I am writing mostly about women as sex workers here, but to be fair, that's what I know best. Someone else can and should write more about men.