I understand your more emotional/intuitive sentiments, while also sharing your more intellectual realisation that there should be a strong and clear argument for deciding that something is a sin (materially, of course, not subjectively, which is a different area).
Personally, while I appreciate the form of the human body and I can be on the record saying that few things in the world are prettier than a pretty girl (men can be nice to look at too, although full body shots tend to gross me out unless it’s Donatello’s David or something), and there’s nothing at all inherently sinful in that form or even in the appreciation of it, I tend to become uneasy when it involves sensuality. Also, while I could look without feeling guilty (and without really having even a temptation to sinful thoughts as long as it really is art), I would definitely feel shame if, let’s say, my hypothetical daughter or wife decided to pose. And I say this as a person who doesn’t attach much weight to similar considerations when it comes to medical procedures or circumstances such as rescue actions or even misfortunate accidents like someone thinks he or she is alone in the room etc.
…But I just can’t understand the motivation to pose on purpose, the necessity to show the resulting image to other people, or the justification for involving the photographer, painter, sculptor or some other person in the intimacy and inevitable sensuality of the whole thing. While it’s great that we can rise above prudishness and behave like normal people when it comes to aforementioned necessities or accidents, I am extremely distrustful towards the need to show one’s body to others (even on the premise of realising that it has some aesthetical value for others). I would be inclined to want to analyse that person’s mind for personality issues, traumas etc. and some form of determined behaviours.
This said, it’s different when the photographer is of the same sex, the shots aren’t in the nude and aren’t intended for anybody other than the spouse to see. (Although it’s perfectly possible for e.g. fully nude ancient statues to be less scandalous than modern underwear advertisements.)
But still, while there is the easily noticeable issue of exclusivity of the spouses (not very long ago it was still spoken of in terms of “right in/to the body”), there is also the problem that we are not supposed to find excitement in our imagination or fantasies–even of our spouses–but only in real contact. I don’t really want to be too far on the prude side here and insisting to make it wrong at all cost (let alone massively wrong), but I don’t like it really. I mean, maybe if it’s mostly aesthetical or sentimental value. But otherwise it renders me distrustful, triggers some primordial fears or anxieties. I was dead pale when one of the most recent special ladies in my life made a joke about such photographs. I basically freaked out and it let to an emotional confrontation.
Therefore I understand how you feel. And I think if it gives you such emotional distress, perhaps you should give up on that part of your practice as a photographer.
I’m a translator (went to law school but mostly changed fields since graduation) and there are contracts I don’t take, even if I can’t really put my hand on a very concrete violation. Some time ago I excused myself and told the agency lady that the descriptions in a perfume ad were too suggestive (they were far on the primordial side) and I’d feel uncomfortable as a person with a rather conservative Catholic mindset. She was understanding and didn’t hold it against me or understand me as outright criticising the material I was supposed to work on.