It's not simply about lust but about propriety. Even if there's no lust involved, there's a natural sense of modesty that people have. Of course, that seems to be tied to Original Sin, but we are, alas, in Original Sin. So, for instance, even people of the same gender would have their modesty offended by prancing around in front of one another nude.
So, for instance, if I saw a painting of Our Lord in which He was depicted fully nude with very detailed, anatomically correct, and graphic depiction of His private areas, I would find that to be blasphemous due to the indignity and impropriety to which He would thereby be subjected ... despite the fact that there would obviously be no temptation to lust on my part as a result of it.
Where is the line? I would say probably at what are called, for a reason, the "private" parts. Breasts have always been a little borderline whether they're strictly private parts (since they're not genitalia), but while perhaps a dignified picture of Our Lady breastfeeding would be acceptable, a picture of her with, say, both breasts exposed, or, worse, entirely nude, would be blasphemous ... even if put on display among women, for whom it would not incite lust, or even if it were in some abstract style, such as Picasso's.
This is probably worth pondering, but modesty goes beyond lust per se. There's a shame and an indignity attached to nakedness (even among people of the same gender) that goes beyond lust. Perhaps it's related to it somehow, but I speculate that it has more to do with the fact that in our fallen state, the body is no longer completely subject to the soul, and it takes on the aspect of a crass materiality that brings shame to our higher spiritual faculties. That's why I also believe that people are ashamed of sins against purity (even in weakness) more than they might be of sins with greater malice. We in our spiritual aspect are ashamed of our animal aspect, but that's due to the fact that these two aspects, the spiritual and the animal, have ceased to be perfectly integrated after the fall.