I am posting the words below as my opinion only, and I welcome hearing from people whose opinions differ from those presented below.
One distinction that many people seem to struggle with is between what is lustful and what is healthy natural appreciation for the opposite sex is either looking at someone in person, or in artistic representations of a person.
I think the term lust is sometimes used too liberally to describe any attraction to the opposite sex, or appreciation of their physical attributes. Lust, as I understand it is an animal instinct, a craving or immoderated desire for sexual relations that overrides ones ability to relate to a person on a personal basis. It reduces people to nothing more than objects for physical satisfaction, and the driving force of the instinct impairs the ability for a deep authentic interpersonal relationship.
This is why pornography is unhealthy, because it reduces people to nothing more than objects for instinctual gratification, and trains a person to feel sexual instincts solely driving them toward the opposite sex rather than a deeper attraction to the person as a human being with a personality, intellect, and life of their own.
However, not all attraction to the opposite sex, or physical appreciation is lustful. This seems to be where many people are uncertain of whether their actions are sinful or not.
I described a sinful attraction above, but we were created for male and female to attract, so attraction itself isn’t sinful.
If for example, a man looks at a woman, and only thinks of her as a prize to be won, or how to act around her so she will be willing satisfy his desire to please his physical drive to have sex with her, he is acting out of lust.
However, if a man looks at a woman, and appreciates her physical beauty without feeling a strong instinctual drive to have relations with her, he is probably not sinning. If he calmly appreciates her physical body as a component of a human composed of body, mind, and soul, as an outward representation of her humanity, and a symbol of the beauty of our human nature, he is not sinning.
An example is the portrayal of the human body in art. There is nothing wrong with an appreciation of even an unclothed body. The body itself, and the art isn’t what is sinful. The situation portrayed has much to do with whether an image is aimed toward provoking lust or not.
For example, suppose there are two photographs or paintings that depict an unclothed person. Does the fact that within the picture is an unclothed person make the picture pornographic? No. All we know is that the human body is shown, so there is not pornography shown so far.
In the first picture, suppose it is a natural setting such as a field or ocean shore. In the nature scene, a couple unclothed humans are depicted. They are simply standing, or walking together. Whether they are clothed or unclothed seems to make no difference as to the morality of the picture, because it is a setting where the nudity is not out of place, and suggests nothing of an appeal to increasing an instinctual drive. Because nudity in such circumstances is not abnormal, it doesn’t seem likely that someone would interpret the picture in a way that draws attention to any particular part of the body in a way that is sexually suggestive.
In another picture, suppose nudity is depicted in circumstances where it seems forced into a situation it should not under normal circumstances take place, such as in a public business, by an exhibitionist seeking attention for doing something abnormal. In this case, the picture is immoral because, it is not simply a normal depiction of the human body where it is appreciated in a normal setting, but a forced attempt to draw attention to parts of the body which otherwise would be covered. In this setting, normally the body would be clothed. Since the picture is depicting an abnormal setting, the focus of the picture would be on what is abnormal, and thus the emphasis is on certain body parts which are abnormally exposed. This would lead to additional thoughts and conclusions about the picture. This is what I would consider pornographic, because the emphasis isn’t on simply a person, but on particular parts of the person, and in a way that is meant to provoke instinctual reaction.