I've been thinking about it a bit recently because of a series on BBC Radio I listend to called Addicted to Sex. I can post a link if anyone's interested but it should be quite easy to find. My own attitude to porn is that it's like junk food - sometimes a big greasy bag of chips is just what you need, but having it all the time means you will cause yourself problems.
Here are a couple of quotes from the programmes. I re-listened and typed them out (because I can type very fast, using both hands:
(this man was edging using porn for about 25 hours a week)
"As you do more and more pornography what tends to happen is that other things in life become less interesting; physical sex becomes ... not interested; humans become less interesting; whenever you want what you believe to be some kick or thrill, fun or whatever, you turn to pornography. And then it becomes habitual, it becomes a monotonous process and that wastes an awful lot of your time and leaves you feeling dreadfully unmotivated and dreadfully isolated and it really is a battle at that point to get out of bed in the morning and I think I'm one of millions around the world who have a problem with internet pornography."
(and this is a woman who was addicted to porn)
"you are in control of every aspect of your pleasure and it gives you orgasms you can't have with another human being."
"porn sets such a high benchmark for the physical characteristics of men (some details follow about penis size) .. expectations are different after watching porn, and you see the way they do it, their abilities, it's endless, the ability to deliver an orgasm, the knowledge, it's all at an advanced level."
"what I was watching became a new benchmark for me and the expectation, the men, sixpacks all the fit bodies, after that, when I would see someone with an ordinary body, so for me personally I wasn't attracted to anyone who wasn't really fit."
Whether a person can be addicted (in the medical sense) to porn or sex is debated but it's clear that it can stop a person from having sex, even when sex is available.