Jesus said that anyone who looks at a woman lustfully has already committed adultery with her in his heart... and maybe every man present (and every future man) felt a good bit uneasy. If adulterers have no part in the kingdom and lustful thoughts qualify as adultery, won't the kingdom be lacking the important ingredient of citizenry?
However we answer that question, we must admit the problem of lust (apparently real even before the invention of the internet) has only gotten worse. As I type this I've just been looking at a long list of terrible statistics about the American addiction to pornography. I won't share the stats because we all know they're bad. Will Willimon states that "lust is an ancient malady, but one could argue that technology enables this sin to be worse for us... the Web is peculiarly well-suited for the exercise of, and encouragement of, lust, which tends to be one of the most secretive of the seven."
I'm not sure, though, that the problem is much worse now than it used to be (after all, the NT addresses the problem of lust and sexual immorality often, seemingly because it was indeed a problem). But certainly the secret is out. Our culture wants us to lust (for sex and for sales) and we find it difficult not to oblige.
I'll make an embarrassing admission. Though I've personally never struggled with pornography, I did, into my early 20's, keep a computer file full of pictures of pretty actresses. It was pretty innocent by most standards (they were all fully clothed, these weren't even swimsuit type pics). But what did God think of that file on my laptop? Why did I have it? In the end, I realized that it was keeping my focus on surface level beauty and could have become, were it not for repentance, a first step toward pornography.
If we are going to learn not to lust, we will have to identify and defeat dangers in their introductory forms. Sexually immoral people don't wake up after 20 years of faithfulness to their spouse and decide to commit adultery. More often, adultery comes after a series of decisions that tolerate a little 'yes' and a little 'no' to love.