I don't think it does either. In some cases it can be though. For example a "director" propositioning someone they knew was financialy in serious bother and what not. I wouldn't link them but thats the stance that my friends that are against the whole idea of porn take. Exploiting women should have been in inverted commas.
That's part of it. It can come out in different ways too. There was a big fuss among the "suicidegirls.com" crowd when they realized that a co-owner of the company was male. Models left the company in herds after discovering it. Of course, it leads you to wonder what the difference is between a woman owning the company and a man owning the company... it makes you question just how much the models actually cared about "equality".
(There was a mass exodus some years before that one as well, but that was mostly a reaction to the overall culture of immaturity and hipsters/scenepoints/image-obsession/treating music as fashion.)
Some people argue though that the sex industry is actually a case of women exploiting men, women using "what they have" to exploit the "needs" of men for money. I'm not sure I agree with it, given that they're usually handing over a big part of their paycheck to someone else.
Even if it doesn't exploit women, it encourages society to treat women poorly. It presents an unrealistic image of the human body. It presents an unrealistic portrayal of what sex is like. It almost always presents women as submissive. It almost always presents women as achieving climax without realistic "encouragement". It also tends to present sex as unemotional, and it treats women as objects.
So to my mind, it's less appropriate to say that porn is "exploitive" of women and it's more appropriate to say that porn tends to corrupt social values by spreading misinformation.
Hmm, you do know how much those pornstars get paid, right? I'd kill to be a pornstar. If that's exploitation... baby I don't wanna be... unexploited 
Strippers can make $200,000 a year if they're successfull. It's entirely possible to retire early and be set for life if you save your money.
Still bet it'd pay better than most jobs... The only way I can see that would be real exploitation would be if the woman was forced to do it for next to nothing.
But that's more televised rape than porn 
Actresses are abused in certain ways, but I don't want to get graphic. I'll send you a PM if that's ok with you.