Most trends operate by reaction, and the common reaction among those who have lived too far down the impossible rabbit hole of "Women must dress in a way that prevents men from lusting," tends to be, "Women should wear whatever they $&*# well please." Which is an understandable feeling, but clearly a reactionary one and therefore limited in scope by its own reaction.
What I would like to do is set that whole sloppy mess aside, and start off in a different direction. I shall not attempt to address any theological questions; I am not a theologian. But anyone with a place to sit and a space to stare off into can philosophize.
So the question I would like to ask is, "What role does clothing play in the expression of our sexuality?" (There are, of course, plenty of other reasons for clothing, such as sunblock and tick protection and frostbite avoidance, but those are far less philosophically interesting--although the fact that our hides are such tender things that we need clothing for such uses is itself suggestive.) And, more specifically, "What and how should children and young people be taught on such matters?"
First, one can note that in any culture or setting where clothing is worn at all, there is universal agreement on what areas are top priority for coverage. (At least as far as I've heard of, and though not an anthropologist, I was a pretty diligent student of back issues of National Geographic as a child.) There's not a culture where kneecaps or necks are strictly taboo but reproductive organs are appropriate for public display. What extra areas come in for mandatory coverage is, of course, widely variant, but the basics always come down to the same.
Why? I suspect is that this is a way we demonstrate our humanity by distinguishing ourselves from animals. Truth be told, the mechanics of mating and birthing are pretty similar for humans and for other mammals. Which is exactly why we, as humans, must undertake them in distinctly unmechanical ways, surrounding them with ceremony and even secrecy.
It is analogous to the way we handle eating . . . we could simply grab and stuff whatever semi-edible substance was lying around, but doing so is considered an act of desperation, not humanity. We surround our eating with ceremony, we prepare our food. There is a huge variety in what ceremony and in how we prepare, but a great universality in our need to make a simple, animal act into a production.
And it is the same way with our reproductive capacity. Various efforts have been made to approach such matters in a simple, natural manner--and they inevitably fail to gain any sort of traction. Nothing is so unnatural to humans as acting natural. Try as we might, we must have ceremony, and if we do not pursue beautiful ceremonies, we will have ugly ones.
What we want and choose to wear is, in many obvious and not-so-obvious ways, influenced by our culture. At the same time, culture is us. It comes from many things, all interacting and playing off each other: universal elements of human nature, practical matters of our geographic location, history and experience, and individual choices. A huge variety of the panoply of human cultural choices are morally neutral: simply part of the beautiful diversity of human experience.
But the fact that many variations are morally neutral does not mean they all are. (Was foot-binding a practice without moral significance?) And the difficulty in drawing a bright-line test does not mean that there is no better or worse to be found. Beautiful music is hard to define, and impossible to fully quantify, but that doesn't mean it doesn't exist or that there is no distinction between fingernails on a chalkboard and the Moonlight Sonata. (Or, for that matter, a simple folk tune sung without accompaniment--I am not arguing for elitism here, in music or clothing.)
To move from this to the practical level, I am willing to bravely stand with most of humanity and endorse the cultural convention of covering reproductive organs in public. It's a good rule. It acknowledges that we are not just animals, that we need ceremony and privacy. It applies with equal force to everyone out of infancy, regardless of age and gender. It is about dignity, not shame.
On a matter more specific to cultural trends, I think it's a reasonable extension, which I will impose upon my children, that one should attempt to prevent one's underwear from being visible in public. (While acknowledging that accidents happen, especially when one lacks hips.) Occasionally it becomes fashionable to ignore this convention, but, honestly, I think it's a bad fashion. Defiance for the sake of defiance, usually, or simple carelessness. The tricky thing is that some clothes may be quite adequate for this task for some movements and not for others. But there's nothing inherently shameful about pointing this out, nor even gender-specific: men in kilts should be careful how they bend over. Anyway, squatting is better for your back.
After that, things get fuzzier. Yet I don't think that means that every ideal must be thrown to the wind. We are getting into areas of art, not science or law. But art can still be informed by what is good. It seems worthwhile to suggest that scraping the bare minimum of propriety is seldom a beautiful act. There is both a beauty in subtlety and a wisdom in realizing that one cannot have the fun of breaking taboos forever. Pleasures are to be savored, and overindulgence in anything leads to boredom even if nothing worse.
Especially for the young, dressing is often part of the worthy and important task of attracting a mate. As a human pursuit, this needs ceremony and discretion, without attempting to entirely deny the bald physical reality.
While the often-repeated claim of modesty culture that "men are more visual" may be groundless, it is a scientific fact that the reproductive capacity of women is advertised in a very visual way: there is, for instance, a high correlation between hip/waist ratio and fertility. For this reason, women's clothing choices inevitably have a more direct sexual connotation than men's clothing. (Women may admire a nice set of pects, but they are not sending us any subtle messages about sperm count. We have other means of detecting a good mate, such as smell.)
Dressing to express one's sexuality while maintaining the decorum and subtlety that acknowledges that human mating is about much more than physical desire is, again, an art, not a science. But it is an important art, and one that should be taught, not simply left to the experiments of fourteen year olds in the first flush of newly discovered hormones. (Like most arts, it is best learned from its practitioners, i.e., more experienced women, not its audience.)
So you will notice I am not going to make any further judgment calls about which specific fashions are appropriate or inappropriate. For one thing, I don't need to make that for the world at large (though I retain the right to make it for children who are clothed with my money). There is a great deal of variation possible and I am not the fashion police. But the lack of hard and fast rules does not have to mean the lack of ideals. Aesthetics may vary in application, but deliberately denying the human need for beauty, for privacy, for ceremony is inhumane and, for that reason, immoral.