I think if you were to try and make beautiful images of naked women you would understand the difference better. The fact is it's very hard work, for the photographer and the model. As the photographer you're working out lighting and exposure and focus and making decisions about composition, and thinking about all of this from moment to moment. You are thinking about stuff like whether to move x out of the background, or how to get the pose you want without the electical socket intruding into the shot. All of this before you think about styling, make up, etc. And before all of this, you are relating to another person, whom you (generally) like and admire .. she is being incredibly brave and trusting with you .. and with whom you are working, the two of you together .. how do you communicate what you're looking for, for the shoot overall, from shot to shot .. There really just isn't time to think about your knob. That isn't to say it isn't fun too. For the model, likewise, there is a lot to focus on .. she has a whole set of concerns, some the same as the photographer's, some of her own, especially if this is someone she hasn't worked with before. Modelling is making yourself vulnerable in lots of ways, from physical risk to the possibility of images being made public that show you in a bad light. When it goes well, there is a rapport there, which can become sexual if the two people choose (as in any working situation a sexual attraction can develop) but actually that is very rare. When you look at the images (often model and photographer together) you are not looking for the sake of arousal, but, usually, for other things.
Also bear in mind that photographers are wired a little differently. I remember at college shooting some nudes in large format, and working with one of the technicians to make a good print. This was a very beautiful young woman naked in a bathroom, and we spent five minutes enthusing about the quality of the print, about how much detail the LF negative could hold, and for much of that we were preoccupied with the chrome U-bend underneath the washhand basin. This kind of thing is actually not at all unusual.
Above all, what you learn from this, the whole process, is that it is actually possible to work in close proximity to an incredibly beautiful woman who is naked and doing what you ask her to do, and not have a hard-on, or be thinking particularly about sex, in fact, sometimes you are consciously avoiding the shot being 'explicit' because that would just draw the viewer's eye to an aspect of the image which isn't your focus. To put it the other way around, it is possible to work with a naked woman and enjoy her, and the images that result, for their beauty. Of course sexuality is present, as it is in pretty much all of life, but oddly, it often has a more natural place in nude photography than it does elsewhere .. just simply a natural part of the world. Often, although you want to have sexuality in the image, you are trying to convey something about it which is not specifically 'erotic' .. you are trying to convey the sense of trust, or fun, or even silliness, or the power of beauty itself, or the physical and emotional strength that is part of grown-up sexuality.
Porn is in the eye of the beholder, primarily. We know that because we know that absolutely anything can be fetishised or sexualised. We know it from reception theory in literature and art. We know it because we see the different reactions of differenet people to beautiful photographs. We know it because, as photographers, we know the kind of questions we ask of other photographers .. what size was the lightbox, how did you meter that ..? We know it, wonderfully, because we are often fortunate enough to become friends with the women we shoot (and their partners) and we see how it is part of a healthy, natural life that is not actually at all obsessed with sex.
We also, many of us who have a religious background, know that traditionally, at least in the Judeao-Christian tradition, the cultural attitudes to nudity, although they are given religious justification, actually have origins outside of the tradition. In the bible, for instance, nudity is not associated with wickedness. Enforcing nudity on someone is wicked, having it forced on you is shaming, but not nudity itself. Sexual immorality is actually associated not with nudity, but with dressing to seduce. Clothing is a big moral issue in both the Jewish and Christian bibles, but is is decadence and self-indulgence .. or materialism or blasphemous arrogance .. that is condemned. From a moral point of view, a high-expense fashion shoot for a glossy magazine, designed to sell overpriced clothes on the basis of their association with a wealthy lifestyle .. that stinks in a way that even the most crude pornography doesn't.
At the end of the day 'pornography' is just a superficial label. At best it indicates an intent, a commercialisation of sexuality, or a depersonalised sexuality. Even so, it is often beautiful in spite of that intent. Bodies and sexuality simply are intrinsicallly beautiful and they survive as beautiful even in images made by people with bad or destructive or exploitive intent. The badness of pornography is what it does to (some, a few) of the people who make it but above all in what it does to the consumer, and it is the consumer who chooses to see in it a means to self-gratification that comes at someone else's expense, who in so choosing is blind to the fact that his pleasure is actually being achieved at his own expense .. financially, and in the way he is being diminished as a human being.
Sadly, few of the people who point the finger at 'pornography' are willing to take the time to think about any of this stuff, and most of us, photographers and models who try to make great images of women, although we try from time to time to explain, really don't see it as our mission in life to educate the wilfully recalcitrant or self-righteous world of middle-class morality or degraded religious sentiment. Those of us who are Christians who do this are fed up to the back teeth of being told, in the name of Christian morality, 'the bible says' .. and when we ask, ok, where does it say it, the answer is 'errr .. ' or some version of 'but the bible says', to which we again ask, where .. and again get 'err ..' and eventually, if we persist, we get 'well I just think ..', which we knew all along, and which I suspect the critic actually knew all along. We all know, those of us who have been round this particular loop, and the critics too, that although it's been revealed by a couple of simple questions that the critic's condemnation is rooted not in anything religious but in their own personal prejudice, nevertheless, the critic will not go away and think about what they believe and why, they will just find a new target for their unthinking predudice.
All I can say is, you try doing this stuff, and you will discover just how difficult it is, and perhaps that will help you begin to actually think about these issues, instead of simply indulging yourself.
This was in response to comments by KeizerHarm made in response to a stunning image by Photorotic: