Pay a woman for sex, it's illegal.
Pay a woman for sex while a camera is running, it's legal.
There's a difference. In a porn flick, the woman gets paid to look like she's enjoying the sex, whereas with a hooker... oh wait. Nevermind.
Both prostitution and the making of porn movies strike me as a perfect example of the free market in action. There has always been a thriving damand for erotica (ranging from romantic fantasy to hard-core stuff). There always will be. Given that demand, and given folks willing to provide the energy to supply the product, the market will address it.
I personally don't mind either one being legal (I'm a small-l libertarian so that's not surprising), although I would personally counsel any young woman to avoid that choice as a career, even temporarily, since it carries high risk and doesn't enhance one's spiritual awareness or reputation worth one damn bit, Asia Carrera notwithstanding.