I'm not an art historian, so it's possible that some of the my generalizations in this post are overgeneralizations.
1. Standards of modesty are culturally relative. At one extreme are Muslims. To my knowledge, Islam even has nudity taboos about members of the same sex. They also have hangups about showing skin in general. At the opposite extreme is Classical Greek and Indian art. Some Indian statues are overtly erotic. There's also the phallic symbol (lingam). Greek art is characterized by both male and female nudity. Some Greek art is explicitly pornographic. The Greek cult of homosexuality presumably contributes to the prominence of the male nude in Greek art.
2. Christians agree that modesty is a virtue, but disagree on what constitutes modesty. It's ironic that Catholic theology makes a big deal about concupiscence, yet nudity is a prominent theme among Catholic sculptors and painters. In addition, Marian iconography gave Catholic artists a pretext to paint gorgeous women. Likewise, artistic depictions of martyrdom are sometimes an excuse for sadistic eroticism. Since, moreover, a lot of Catholic art was sponsored by popes and prelates, it can't all be chalked up to randy laymen.
3. Botticelli, Bernini, Raphael, Ingres, Renoir, and Dante Rossetti are artists paradigmatic for celebrating the female form. It's my impression that generally speaking, French and Italian artists celebrate physical perfection (especially female) in a way that many Northern European artists do not. English artists split the difference.
One reason may simply be that warm sunny climates are less inhibited about exposing skin than chilly climates. That may be a partial explanation for the exuberant nudity in Greek, Roman, Italian, and Indian art.
It's amusing that after his "conversion," Botticelli switched from Classical to Christian themes, yet his Madonnas look just like the women who populate his Classical paintings. The setting and outfit has changed, but the women remain the same! Nothing wrong with that. Beauty is universal.
Although Rembrandt paints nudes, they're not beautiful women. Rather, they're the women he loved.
4. In Christian art, male nudity seems to be more confined to depictions of Adam and the Day of Judgment. You also have artists like Michaelangelo and Eakins. That raises questions about their "sexual orientation"–although Eakins also did female nudes.
5. To my limited knowledge, skinny-dipping was the norm until the Victorian invention of swimwear, although I assume it was usually sexually segregated. The public Roman baths were unisex, but that reflected pagan mores.
6. In traditional Western art, there seems to be a tacit code about pubic hair. Artistic nudity is permissible so long as pubic hair is brushed out. I don't know the rationale for that convention. Was it an arbitrary custom in which pubic hair was deemed to be too realistic and therefore obscene, whereas full-frontal nudity was permissible so long as the artist omitted that detail? Or did it trade on the "innocence" of prepubescent nudity? Of course, if the nude model is evidently sexually mature, then that's a ruse.
One exception to this unspoken rule is a 5C Byzantine ivory diptych of Adam in paradise (in the Museo nazionale del Bargello in Florence). Perhaps that dates back to a time before the later tradition became entrenched.
7. In Christian ethics, the notion of modesty revolves around the concept of lust. Standard prooftexts include Prov 6:25, Mt 5:28, Rom 1:24,27, and 2 Tim 2:22.
i) In context, Prov 6:25 refers to prostitution
ii) In context, Rom 1:24,27 has reference to homosexual attraction (and behavior).
iii) In 2 Tim 2:22, does "lust" refer to something in the mind (attraction, imagination) or behavior? In the 1C Roman empire, sexual immorality covers premarital sex, extramarital sex, promiscuity, prostitution, rape, incest, sodomy, lesbianism, pederasty, and abortion. Christians were obligated to foreswear that behavior.
iv) Mt 5:28. This is the locus classicus:
a) A problem with the traditional interpretation is that lust comes in degrees, so on that interpretation, the text offers no concrete guidance on where you cross the line.
b) In addition, the traditional interpretation has been challenged:
8. Of course, lust can't be entirely detached from sexual misconduct since that's the motivation. They are asymmetrically related. It's possible to lust without acting on your impulses, but lust provides the incentive for the corresponding behavior.
9. Then there's the question of how to define lust. Consulting a Greek or Hebrew lexicon isn't the answer, since that will simply give you an English synonym. One issue is whether the concept of lust can be determined by Scripture, or whether Bible writers expect the reader to have a cultural preunderstanding of lust based on human experience, especially against the pervasive backdrop of heathen sexual mores.
10. There's the additional question of whether there's a more restrictive (indeed, exclusive) standard for married couples than for singles, especially in the realm of the imagination. Sex outside of marriage is forbidden for both groups, but what about art or fantasies? The alienation of affections is a danger in marriage.
11. Modesty is a broader category. Take Christ in the House of His Parents, by Millais. That was quite controversial in its time. It offended conventional Victorian piety. Not because there was anything slightly erotic about it, but critics considered it an indecorous way to represent the Holy Family. Too down-to-earth. The hostile reaction reflect the artificiality of some religious sensibilities.
12. In Christian theology, the human body is both a divine gift as well as God's handiwork. A marvel of engineering. Man is the apex of creation in our solar system. Perhaps in the entire universe. If it's permissible to make artistic depictions of lesser things in nature, why not the greater? Take athleticism. When I watch joggers, some men and women are natural runners while others are manifestly not. They have no idea how to hold their arms or coordinate their arms and legs. By contrast, young runners with innate coordination have an elegant gait. It's enjoyable to watch the natural grace of a good runner. Beauty can be simple. Wildlife photographers take pictures of cheetahs chasing down antelope. It's exhilarating to watch. Art it motion. And of course, we have a special affinity for the human body.
13. Even if we consider artistic nudity to be permissible, there are ancillary issues. Take Renoir's Les Grandes Baigneuses. Consider the girl in the water, painted from behind, who's splashing the women on the riverbank. She appears to be in her mid-teens. Fresh ingenue beauty, projecting playful, unaffected innocence. But I assume she was a real person, like the other two women. What's the fate of models when their springtime bloom wears off? What happened to that girl? Did she die of old age? Did she die young, from TB or influenza? Did she die in poverty? Did she contract venereal disease and die on the streets? In the painting, she's frozen in time, in the flower of youth, but she lived and died. Do viewers every wonder what became of them? I'm reminded of Anton Chekhov's short story about the model exploited by art students and medical students ("Anyuta"). Used, passed around, then discarded. That's fictional, but based on real-life examples.
