If you are trying to convince readers that women are an important part of war in an alternate world, is sticking them in costumes designed to titillate instead of protect damage the worldbuilding credibility? How could you balance the rule of cool with realism?
Yes, it can and most likely will damage the credibility if you are aiming for a serious setting.
"Boobplate armor" as you put it would be highly dangerous and useless. Using it will automatically show every reader that has ever done a tiny bit of research into this topic or had some experience with armor that you are aiming for the generic anime/fantasy game genre.
Especially if women are important they should be defended as best as possible.
That being said: this is mostly important in visual media. I can't remember reading any description of such armor and I've certainly read a lot of fantasy books. Maybe it just slipped past me, but I would find such a description to be very weird.
If you are going for a high-power-magic setting you could just describe your female warrior wizards in normal clothes or something like that to show how mighty they are and that they don't even need armor like those poor men without access to magic. I wouldn't try to sexualize their armor. Ever. (Of course it's your decision as the author, but I think you are massively damaging the realism and it's not even cool in writing.)
I generally agree with @Secespitus and that may be all the answer you need to consider.
However the idea of sexuality as a weapon is an interesting one, and could potentially be done well in fiction. (Probably has.)
There is some (speculative) thought that prostitutes were used to compromise the sitting US president. No boob armor was involved, but the scenario makes the point that human sexuality can be weaponized.
If you want to explore this idea, as a reader I would want to see both genders used in some such way, not just the sexualization of women, and I would want it to be smart, and that means involving something beyond boobs. There are other things about women that men find attractive and this gets into all the senses. Conversely, women presumably find many things about men attractive, not just their physicality, and this also can be weaponized.
As a reader, I'd also want it to be not-too-much of the story. Maybe a subplot.
In other words, don't play to least common denominator.
If you can give a convincing excuse for boobplate in your fiction it might mitigate how offended anyone gets.
Maybe the female warriors like the psychological effect of emphasizing their gender, either because their culture is staunchly feminist or their enemy's culture is staunchly misogynist.
If the female-warrior-culture is famous for its battle prowess, anything that helps identify them on the battlefield, including boobplate, will be a welcomed addition. Like how Polish hussars wore wings. At best, the wings physically did nothing, and at worst added drag and/or got hung up on things. But they were worn because the hussars were feared, and occasionally enemies would run just because they saw the wings.
If you only need a few of these half-dressed femme fatales, you could make the exposed parts an illusion. The women might be warrior wizards who use a small part of their magic to bring an element of style and sexuality to the battlefield.
The joke is on all the men. These women are fully armored and completely safe from all but the most skilled of opponents, but they look like they are facing potential death with a courage that the mightiest hero couldn't match.
The illusion might also serve a strategic purpose, as the body parts which it appears to leave bare are actually the heaviest armored. By making the enemy think that leg is exposed all the way up, or that cleavage is visible all the way down, they draw attacks to the points where their armor is best prepared to block.
The illusion might even enhance the face and figure of the warrior wizard. Concubines and sex slaves are among the most prized of the spoils of war. Why not make the prize so appealing that any male opponent hesitates for fear of damaging the merchandise.
There is no idea so absurd that it cannot be given a serious treatment. Basically what you need to do is describe its function and practice as if they were realistic elements of your fantasy world.
For example, you can assume in your fantasy world its magic system either through sexualization symbolism or the effects of magic are strengthened or made more powerful when coupled with sexual symbols. To avoid the obvious tittilation this should also apply to men as well as women. If the women warriors wear boob-plates, then the men warriors will sport massive cod-pieces.
Because the magic system relies on sexual symbolism, the boob-plates don't need to be worn over large bosoms. They only need to suggestion the shape of breasts to communicate magically female power. Likewise the giant codpieces of male armour is there to suggest potency and male strength.
The simple answer is treat the concept seriously. Show the characters behaving as if the concept is valid and real to them. Make them respond to it seriously. You are definitely walking the tightrope by playing with an inherently absurd and, frankly, sexist concept. But if you aren't doing it for laughs and your story demonstrates it has a valid reason for being part of the story. Also, show it also applies to both males and females. Simply play it straight. Provided the rest of your story and its world are presented in all seriousness, then you have a chance of succeeding with it.
Strap on your armour with its cod-piece and show the world its power. Write with conviction and be unafraid.
I say it depends on the type of boobplate.
As @Alexander mentioned in his comment, Greek armour was anatomically correct. If the boobplate offers protection to the whole torso, then I see no problem with it. You can even have the boobplate be extra-sexualised (super-hero films often have the uniforms carry a perfect six-pack and I remember Hugh Jackman mentioning they had something extra in their pants to make the guys look bigger and, therefore, more virile and powerful).
On the other hand, if the boobplate is nothing more than a bikini armour... I love fantasy and I've read tons of it when I was younger. By now, I know that if the writer goes for that cliché, it's probably not worth my time because it is always hand in hand with more cliches of the same caliber.
What is even more depressing is that the bikini-armour warrior is often inserted as a way to attract female readers/viewers when, as far as I know, it has the opposite effect (at least on the female fantasy fans I know) because its real aim is obviously eye-candy for men.
I would, however, love to see sexualised armour as @a4android suggested. If sexualisation has an in-story function, then it stops being pointless eye-candy and it becomes worthwhile.
Note your use of words there.
"Sticking them in costumes designed to titillate"
This is the main problem. I don't mean to put words in your mouth, but it's a very common convention to, as you said, stick women in these costumes. To take a stripperriffic fashion sense and impose it on the women of their world, globally, and almost completely without regard to the individual personalities of the women in question, as if they don't even dress themselves and are quite literally dressed by the author and magically made ignorant of what exactly they're wearing. Trying to design outside circumstances to justify why serious, professional women (and only women) would walk around wearing ridiculous, impractical chainmail bikinis with thigh-high stocking greaves and not bat an eye about this as if it's completely normal is pretty much always going to prompt eye-rolling, because you're not even addressing the biggest problem. Even if it were practical, no woman would dress like that unless they wanted to. To truly justify women going into battle dressed to titillate, you'd have to explain why these women would value doing this, or at least, bare minimum, why they would be so cavalier about brutal, violent combat that the more "outgoing" among their population would see it as a perfectly natural occasion to show off.
And that isn't to say it can't be done. It's just that such societies aren't generally going to be the most "serious" of societies.
For example, my story, which is about the aftermath of modern day humans being given access to a variety of supernatural runes on a weekly basis, has a group of cultists who believe that humanity's sole purpose for being created is for the entertainment of a pantheon of old-school, bloodthirsty, psychotic, hedonistic, incestuous pagan gods who brought the magic into the world as a last-ditch effort to make us stop being so complacent and boring, and will destroy us if we don't shape up. Their entire religious morality revolves purely around being entertaining to their perpetual cosmic observers, rather than being noble, stable or practical. When society inevitably collapses due to all the magical shenanigans, they get even crazier and turn into this stark raving mad warrior-raider culture led by a power-mad prophet who can predict upcoming magical powers, determined to spread violence, excitement and chaos to all who would dare subject the gods to the tedium of their mediocre existences. They devote huge amounts of time to becoming, as best as they are able, warrior-artists, cultivating personas, aesthetics and fighting styles with which not to best achieve victory, but to best please their gods, and cherish these things as the most important things they've ever made or will ever make.
Now, the entire rest of the cast of this story rightly views these people as out of their gourds, and their primary narrative purpose is to be a source of comedy, senseless conflict, and in the case of the stronger members, entertaining and memorable fight scenes. But let's ask that same question of this wretched hive of hedonistic madness: "Why are some of these women running into battle with half of their bodies exposed?"
The answer: because they want to. They are religiously, zealously obsessed with living lives worthy of their own show. Their entire lives are meant to be performances for the gods, and they, personally, as individuals, have decided to please their gods with their sex appeal. They believe their gods would be far happier with them if they fought in sexy underwear and with a chainsaw, screaming and cackling like a lunatic, than if they put on actual armor and fought with an actual weapon. Even if it would mean the difference between victory and defeat. They see caution, pragmatism and boring, sensible dress as a coward's way out that would doom them to the deepest depths of Hell should they dare resort to it to win.
So I guess my point is that if you really, really want to "justify" sexy armor in an otherwise serious story, it's not enough to create a way for it to be a practical combat choice. You have to make us believe that the characters who are wearing it are the sorts of people who would actually want to, whether on an individual or cultural level, and let everyone else dress the way they realistically would. That in and of itself will do wonders for suspension of disbelief, but keep in mind that there's always going to be an air of tastelessness, campiness or absurdity hanging over the whole thing depending on how far you go. Not that there's inherently anything wrong with it, but keep in mind that some sacrifice to seriousness will have to be made pretty much no matter what you do.
Ah, the good old balance between hard sci-fi and creative liberty question. I can't answer all, as each time this problem occurs, the solution differs.
I did create the somewhat useful Mephisto's Solving Algorithm for Awesome it reads out like this:
- Collect everything you know about the problematic item.
- Go through it, and see if it can be solved, real-world excuses/analogies are what you searching for, for example, an analogy can be drawn between the Quetzalcoatlus Northropi and a dragon, at least I think.
- See if you can discard something to save the rest. (do dragons need to be that big?)
This allows you to keep something that's useful within a context, created in this way.
A dragon from Second Earth (my fantasy setting) will nowhere nearly be as powerful as Smaug, but why should he? He won't be pitted against a Gandalf or a Sauron, or a shooty boi with an infinity +1 arrow.
Applying this to your question:
Why do you need boob plates? A 15th-century gothic plate armor is the medieval equivalent of iddqd. Its weight is evenly distributed, allowing you to do somersaults if you wanted to, it also looks cool.