Daughter of Danger - what constitutes immodesty in art?

A while ago, the fourth book in John C. Wright's Moth and Cobweb series, Daughter of Danger, came out. I haven't read any of this series but I want to; but they haven't come out in paper yet, only as ebooks, which I don't read. I await the paper editions.

Anyway, there was a bit of a fuss kicked up over the cover art by Scott Vigil, which some called too risque and immodest. Here it is:

I myself think it's beautiful, and it reminds me of 19th century American patriotic art, like these:

by Mel Crawford

So I want to raise the question of the human form in art and the morality thereof. My mother was an art teacher and is a huge fan of Michaelangelo; we grew up being taught that the human body was a creation of God that artists portray for the sake of its Beauty, and that nudity was not necessarily sexual at all. But there were those in our parish and homeschool group that didn't agree, and since I've started my career as an illustrator, I've been told a few times that my drawing and painting women with legs and cleavage/breasts exposed is a scandal and an evil. (For whatever reason, there doesn't seem to be a problem with exposed chests and thighs of men.)

I do of course recognize that there is such a thing as salacious art meant simply to arouse lust. But I don't think all exposition of the female body is such. In this post, though, I'm more inviting opinion than arguing for my own. (I may elaborate on mine later.) So please let me know what you think!

One thing I observed, however. There were families in our homeschool group that were obsessive about modesty and almost puritanical regarding things like comics, anime, and video games. And it is, overwhelmingly, the children of those families who are now, unfortunately, living in fornication. Maybe it's rebelliousness or in-for-a-penny-in-for-a-pound mentality; of course I don't know all the factors or possible reasons, but it is a definite pattern.