See, I think it's important to make a point before I truly go into this. It's not about being angry about artists drawing pretty women. I don't have a problem with that as such, as I know there is a definite attempt to present aesthetically pleasing people in most art, hence the perfect figures and faces. I also think that argument could be a bit problematic in itself, as attractiveness is definitely subjective, but it leads me onto a point I think I should make. You can have cute and/or pretty women in armour, no problem. They could be the toughest warriors to have existed. But that doesn't negate - on any level - how armour works nor the point of it. Red Sonja's large chest and striking features do not make armour somehow less functional, thus she can go without it, as an example.
So, in fantasy art you tend to have two main styles of art. You have Realistic & Sensible, and you have Fantastical & Impractical, which I'll refer to, respectively, as R&S and F&I from here on. You will also typically find that R&S art is more anatomically correct, whereas F&I is frequently exaggerated. Let me present two examples, of different styles, artists and genres, to give a rough idea of what I mean.
This first image is titled, ahem, "hottie warrior 4" and it's by an artist on DeviantArt by the name loztvampir3, and you can find it here. As you can see, she is wearing armour that is mostly insubstantial, leaves various weak points unprotected, and she's drawn with a disproportionately large bust. In fact, I think you'd struggle to find anyone with a bust that size and shape who hasn't had extensive surgery, and even then I think it'd be a push.
It is, therefore, easy to put this particular piece into the Fantastical & Inappropriate pool, due to the lack of protection and the over-sexualisation of the wearer.
a page about this piece can be found here, on Donato's site) and it shows a fairly average looking woman in what is easily a historically accurate set of armour. It is a sober and respectful piece of work, showing Joan as a warrior - a loved one at that - not as a woman.
Of course, this isn't to say that you can't have femininity in fantasy art. You can, certainly. It is entirely possible, and reasonable, to draw armour with what we would consider 'feminine' touches, although with plate armour this - as discussed elsewhere - is highly impractical and typically a trait of F&I styles. This femininity can be represented any way the artist wants, either in terms of the armour itself or with other touches. I'm going to present another two examples and compare them to show how femininity can be represented in different ways. I'm going to use pieces from the same artist, a DeviantArt member by the name of Ruloc, from the same themed set.
Storm Caller and Kethil Wyvernsbane respectively, and depict two characters that are played by Pathfinder fans. As is clear, both characters are female, but this aspect is represented in two different ways. With Storm Caller, the character's build - though mostly obscured by a steel cuirass - is clearly of a build we'd consider feminine, and this is further compounded by her facial structure and hair style. There is no reasonable doubt that this character is in fact female, yet this is clearly shown without any unreasonable or fantastical approaches. The character of Kethil is represented in what we'd consider a more feminine manner, she's wearing clothing that accentuates her figure and makes it clear she has breasts, and her hair and face again suggest a female character. As she is not wearing armour, the artist - and player - are able to take liberties with practicality, though I'd personally argue the character's outfit is entirely reasonable. But again, this is handled in a modest way by Ruloc, without focusing on any sexual aspect of the character.
This brings me to another point, however. Can armour be revealing, and impractical, without being distasteful? And I think I can happily say yes to that question. This is when it comes down to a matter of taste, and perhaps even respect. I also think one can allow for some degree of handwaving when it comes to cultural sensibilities - after all, some (if not all) Celtic tribes used to run into battle naked wearing naught but their torques and a coating of woad (a dye made from plants), although this would have been a mixture of cultural beliefs and genuine tactics, i.e. putting fear into the enemy. This doesn't apply as much to, say, an Elven ranger in the forests, but there are places where rules can be relaxed a little. The one issue I have, however, is the amount of bare skin typically on show.
Is it not sensible to suggest that the character in question may have a shirt on underneath? Something to sit between the armour and the skin, reducing the chance of chafing or discomfort?
I suppose this brings about a point about context. A work which bends the rules (or even violates them) should be taken contextually. If a character is in a humid, warm forest, it is reasonable to suggest they may wear lighter armours - even show skin - in order to keep cool. If they are in dungeons, caves or even zones of a low temperature (e.g. mountains), it would make sense that they are covered in order to keep warm and dry. They wouldn't reasonably be walking around with bare legs and bare chests - they'd die, essentially. One could argue the same for sandy areas - suitable clothing would be worn to keep the heat and sand away. Bikini armour might be good for getting a tan, but it'd be uncomfortable if the sand got into it!
Finally, I'm going to move on to the 'Attitude' part. To me, this argument is very much a part of the Sexism In Genre debate. It is tied to the way women are viewed, portrayed and represented in fantasy fiction (and, to a lesser degree, science fiction). Without wanting to make huge, sweeping generalisations, I personally find that the vast majority of fantasy art is made to portray women - of any type - in a sexual, attractive manner. It is not about showing how good a warrior she is, it is not about showing her skills, it is about making it clear she is attractive. This can be evidenced by the lack of sensible armours, by the accentuation and exaggeration of the character's busts - contrary to how armour (and breasts) actually work, and by the poses said characters tend to be found in. Whether it's in a comic or general themed artwork, there is a culture of deliberately representing women in these ways, whether they're the heroine or a monstrous creature. They might not have a pretty face, but they'll have perfect breasts (often accentuated in some way) or an attractive figure.
The attitude towards female armour, particularly in fantasy art, really needs to be changed - and now. Women are portrayed, in the majority of pieces, as sexual in some manner. This may be their pose, their expression, the accentuated bosom, or even the armour itself. They are represented in this way at the expense of reason. Revealing armour does not protect - it endangers. Boob-plate armour does not protect - it endangers. Loose hair, loose cloaks and loose clothing do not protect - they endanger. Whereas men are typically shown as bastions of strength and power, women often seem to be shown as if they are merely playing - perhaps why their armour can often look like they should be sold next to the "Sexy Nurse" and "French Maid" outfits. These characters are not treated seriously, and thus cannot be taken seriously. A female warrior would not be concerned about whether her breasts are accentuated by armour, she would be concerned about having her weak points protected - the very point of wearing armour. Armour is worthless if it does not protect, at which point it becomes heavy - and cumbersome - decoration.