I disagree with your usage/meaning of the word lust. Our Magisterium does too, which is primarily why I disagree with you. They teach that, "Lust is disordered desire for or inordinate enjoyment of sexual pleasure. Sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive purposes." (Source found in my post to which you're responding.) In that definition they treat the matters of procreation and physical unity explicitly, and distinguish those things from lust.
Instead of quoting what the "Magisterium" hand down - you should be looking at their arguments and, if they be found good, present those arguments. There can be no real discussion otherwise. Or, if I provide you with a separate definition from another authoritative source - would you accept it? No - so it's better to stick with generally accepted definitions as the basis of one's discussions.
Lust is simply an innate physical desire that we are all born with, and it is a God given desire such that we are led to procreation. There is nothing "disorderly" about it in of itself, nothing sinful. It is rather good and has a clear purpose. That is why God commands wives not to deny their husbands, and visa-versa.
However, we know that just because a desire in one context is good does not mean that the desire cannot be misdirected or that we cannot act on a good desire in a sinful manner. As we know: Good intentions pave the road to hell. Rape, even with the intention of procreation, is sinful. Thus there are more important factors in the evaluation than simply procreation. On the other hand, homosexual relations are sinful as well - so we can't simply dismiss the importance of natural relations either.
Nevertheless, if a woman or man is barren/shooting blanks, that does not thereby invalidate one's marriage or the command that a wife and husband are not to deny each other. Rather, to divorce because of this would be a sin, and to cease having sex simply because one or the other, or even both, are incapable of reproduction would likewise be a sin. Thus marriage and sex are not simply a matter of procreation, though arguably the primary purpose thereof. Marriage and sex also legitimately serve a purpose even in the absence of the potential of procreation: a satisfactory marriage, an intimate bond, etc. Indeed, sex is a major driving force in the forming of social bonds - either directly or indirectly.
Now, back on to the matter of porn - let us explicitly limit ourselves to the matter of self-pleasure, as I believe that is more or less what you and others are focused on. You say self-pleasure is wrong for it serves no purpose. Yet doesn't it? Does it not allow one to satisfy their own desires without, say, paying for a prostitute instead? And it doesn't harm anybody - no illegitimate children or diseases. And if it hurts no one, and goes against none of God's commands, why should you say it is wrong, a sin?
Of course one left with self-pleasure as their only recourse would generally agree that sex with another is more desirable - but one cannot simply rush out and marry whomever. Given this fact, it is better that they resort to self-pleasure rather than illegitimate relations with another.
