I don't know how many people actually pirate porn. As Alterego-X brought up, "titties are post-scarcity". While movies often have massive budgets, casts and sets that clearly identify them and can be used as the basis for copyright lawsuits, anyone can make a porn video or go on the internet and find literally tens of thousands of amateur videos which are mostly indistinguishable from high-budget ones (after all, there are only so many ways to do the deed).
But more importantly, porn companies are cutting edge . They know they can't beat the pirates of the world, so they come at the problem from a new way. They upload clips of a certain length to websites that give them a cut of the ad revenue. Should the person decide to buy the full video, everything is great; if not, the company still gets some money.
Porn is always at the front of new media (like printing, VHS, CDs, etc.) because it is something that nearly everyone will want to get at some point, but that the government does not really want to protect. In a few years, I expect that regular media companies will follow suit (Comedy Network, among others, already does) .