So what do a man and a woman need to do sex and procreation well? They need to focus on the dignity of the other person and how the sexual impulse is a call to love that person completely. They need to avoid thinking about the shortcuts. That is how can satisfy my sexual desire without actually giving myself away in love. But these thoughts are hard to put out of your mind. This is especially true for young people who are in the initial stages moving towards what could be a solid family but could also be a dysfunctional family. They tend to have less self control and so the distractions are going to be more damaging.
When people ask a question like, "What does it hurt if I indulge in some pornography?" there is an assumption that they are an island. The better question is, "How can the common acceptance of pornography make it harder for a couple to focus their sexual energy on each other in a way that is not degrading?" Then the answer becomes obvious. If you are defending an action as moral you can't assume you are the only one who will be doing it. You need to look at the full impact of everyone embracing the same moral principle. You need to specifically look at what it does for marriage and the raising of children. Then you need to be honest. Is it harder to have a strong marriage in a pornographic society? Is it harder to raise children well in a pornographic society? Sure some will reply with "I don't see how" just because they don't want to see. You need to be honest.
There is another level of the question. How does my indulgence effect my personal ability to date in a healthy way and to enter marriage with a healthy view of sex? There is an assumption that because marriage is not on my radar today that I don't need to worry about what I do to my personal sexual faculty. How do you know that what you are doing has not prevented you from developing a healthy marriage with a person in your life? That is what your sex drive is supposed to do. Can you honestly say you are not making that less likely?
That is the key from a Natural Law point of view. Are you making the natural end of sex less likely or impossible by using sex in an unconventional way? If you are then you are committing an immoral act. That is a sin against reason. Natural Law does not make reference to God. It discerns the purpose of things by reason and asserts that to use something against its own purpose is ultimately self-defeating.
You can make similar arguments for all of the other sexual distortions. You can see that if they were practiced freely in society then they would make strong families more challenging. You can see that pretty easily because we have done pretty much all of them in our society. Anything we can do to make marriage harder we have done. Anything we can do to make raising children harder we have done. We have done them in the name of reason and progress. We are just that smart.
The good thing about thinking at this level, that is the level of society and not the level of an individual act, is that you don't have to focus on the precise details of the act. Pornography is notoriously hard to define. Here you don't need a precise definition. You can redo the argument at a more precise level, making clear that we are talking about PG13 rated porn rather than R rated porn. You can still keep the discussion on the big picture. Sexual morals are not about being concerned someone might be having too much fun. We are concerned man and woman might miss out on the great love they are meant to have in marriage and they might be unable to give their best gift to the next generation, that is to be great mothers and fathers. That is what is at stake, our greatest joy and the well being of our children.