Part of the answer appears in Doidge's book, The Brain that Changes Itself, on page 104. For at least some men, "pornography... initially helped them get more excited during sex but over time had the opposite effect." Doidge explains further:
Pornography is more exciting than satisfying because we have two separate pleasure systems in our brains, one that has to do with exciting pleasure and one with satisfying pleasure. The exciting system relates to the "appetitive" pleasure that we get imagining something we desire, such as sex or a good meal...What pornography does is like creating the smell and sound of a fresh, crisp apple (say) or the aroma of a juicy steak -- while simultaneously sewing your lips shut so you can't ever have the apple or the steak.
The second pleasure system has to do with the satisfaction, or consummatory pleasure, that attends actually having sex or having that meal, a calming, fulfilling pleasure.ibid., p. 108
This chapter in Doidge's book is not for the squeamish. It is, however, a healthy corrective for the thought that pornography is a victimless crime. The victims are first the consumers, then their wives/girlfriends, and (though Doidge doesn't say so) to a lesser degree every woman they come into contact with. And Dr. Doidge isn't a preacher or a scold, though he does advocate healthier practices.
True confession time: If I could press a magic button and somehow make all pornography and only pornography disappear forever from the internet, I'd press it. You're probably thinking all kinds of things about what pornography really means, how could you possibly know what effects a particular image will have on a particular man, etc. But before you condemn me as a moralistic scold (I wouldn't be scolding, just censoring), read the chapter -- I mean the chapter in Doidge's book, not some Bible chapter.
Oh, and I'd probably start looking for a button that could remove pornography from any other electronic media because of my own weakness, which I know is not uniquely mine. (The Marriott Corp. knows, too -- it gets them lots of money from business travelers and their employers.)
I'd do it for the same reason I'm against cigarette advertising targeted at kids (or at anyone else for that matter): that a society has a duty to its less capable members to protect them from those who want to exploit them. And don't kid yourself--a man addicted to pornography is no master of himself, any more than some teen-ager is when s/he gets hooked on cigarettes of any kind.