"2354 Pornography consists in removing real or simulated sexual acts from the intimacy of the partners, in order to display them deliberately to third parties. It offends against chastity because it perverts the conjugal act, the intimate giving of spouses to each other. It does grave injury to the dignity of its participants (actors, vendors, the public), since each one becomes an object of base pleasure and illicit profit for others. It immerses all who are involved in the illusion of a fantasy world. It is a grave offense. Civil authorities should prevent the production and distribution of pornographic materials. "(CCC)
OK, I’ll jump into this thread. I would say that the CCC’s definition is the best here (as it should be) as the depiction of the naked human form is not sinful in and of itself. (Hey God created us naked - at least that’s how I remember my kids being when they were born). Depicting the marital act outside of marriage is obviously sinful (and I mean showing people ACTUALLY doing the marital act), since it is a sin against the sacrament of Matrimony and chastity. Also, even if the actors are married to each other, the selling or distribution to third parties removes the intimacy of the act, as stated in the definition.
I would say that nude or semi nude depictions of the human body as it is, in various poses (not depicting sexual activity) is NOT pornography, since it doesn’t infringe on the dignity of the conjugal act. However, that doesn’t necessarily mean that such a picture couldn’t be a sin against chastity if the primary and even sole purpose (i.e. intent) of the photo is to elicit a sexually arousing response (i.e. lust) in the viewer. This would be a sin for the photographer and model. A person who views such a photo may be able to see the beauty of the human form and appreciate the wonders of God’s creative ability, or view it and become sexually aroused. If a person viewing it becomes lustful towards the model, this is obviously a source of temptation and should be avoided. If not, no harm to the viewer.
If the primary purpose (i.e. intent) of the photographer and model is to depict the beauty of the human form, there is no sin against chastity, so long as both are able to avoid impropriety (i.e. lustful thoughts). If niether could do this, obviously they should find other professions! Thus a beautiful nude photograph of the human form can be taken without any fear of commiting sin or being “pornographic.” A viewer of this photo may be able to see the beauty intended by the photographer and the model (no sin), or a viewer may be prone to get excited at the depiction of the nude form and this would be a near occasion for that viewer and should be avoided.
Some posters have said this is relativism - I heartily disagree. Just as one person can look at a cake and eat one piece, so another might be tempted to eat until the point of being sick (Gluttony - a mortal sin). That doesn’t make the cake evil, or the baker a sinner.
Likewise with medical professionals (especially OBGYN’s) one must look at all the aspects of sin to determine it (full knowledge, intent, and consent.) There may be doctors (and some have been prosecuted) who take advantage of their position to exploit patients. This doesn’t mean that the practice of obstetrics is sinful, or that a woman receiving a pelvic exam commits a sin, per se. It could be if she intended to derive illicit sexual pleasure somehow, with full knowledge and consent. (however unlikely)
We are each called to a deeper self-knowledge. And while certain behavior can be objectively identified as sinful (fornication), whether a specific photo of a person, whether clothed or nude is sinful has to address all the components of sin.