Porn tends to focus on pushing the envelope. How far can we go, how depraved can we get—it can get to a point where you're so overstimulated, nothing is enough.
Ordinary sex, just a man and a woman, a penis going in a vagina, might not do it for you anymore. Getting turned on becomes an elaborate formula: kinks + depravity x infinity. At a point, even the filthiest porn can fail to register as exciting. Like overusing a potent drug, the level of filth it takes to feel something, anything, can keep elevating to the point of eventual numbness. And porn can react with your brain in the same way addictive drugs do.
The most troubling part about this isn't what this can mean for your masturbation habits, but what it might mean for the women you're having sex with. Will she be able to turn you on, or will you be losing your erection when the sex can't compete with the images you've seen on your computer?