If I had to teach my religious ed my view of why porn is wrong, here's one way I would do so:
Have you ever hung out with somebody, gone to their house because they had a pool, or a plasma tv, or maybe let them buy you stuff but you weren't really their friend? You didn't really like them, and, well, basically you were using them? You got your kicks and had some fun but it was phony and fake, wasn't it. Because it wasn't borne out of true friendship and it demeaned the person you used, but it also demeaned you, too. And it would be hard to put your finger on it, but you know you felt it was wrong.
Porn's like that, too. You are using somebody for your own private kicks; they aren't really yours; and you kind of feel demeaned for doing it.
Only God can give you the person to enjoy in an intimate way. He gives it to you in trust: it's marriage. Other than that person, nobody else belongs to you in that way.
V's wife.
I'd go even simpler. Your goal in life should be to find a relationship with a suitable woman and marry her. Once you do that, you will find intimate relations with her to be extremely pleasurable and fulfilling. Nothing else in life can compare with the fulfillment you will get from a good marriage.
There are easier ways to get some of the pleasure, including looking at pornographic pictures or videos, or picking up women for one-night stands. Those ways will give some of the pleasure, but won't give you the fulfillment. The more time you spend seeking immediate pleasure, the harder time you'll have finding a fulfilling relationship.
Note that it's not necessary to bring in any religious or moral overtones. Even someone who's only interested in himself and cares nothing about morals will still benefit from the message above. It's not necessary to punish people for succoming to temptation; just let them know what they're missing by doing so.