The chainmail bikini is not inherently sexist. One of the examples of this is Conan the Barbarian, who runs around in leather underpants. His character is a bit campy though, and doesn't represent the fantasy genre overall.

Overall, the fantasy genre has serious issues of sexism. You would often see a man in realistic armour alongside a woman in a chainmail bikini. The first D:OS made this exact mistake.

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So yes. Historically the chainmail bikini was introduced to the fantasy genre in a highly sexist way.

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Feel free to give elves a racial talent "unable to wear heavy armour". Maybe they have to instead use some kind of special elven magic in order to protect themselves while wandering around half-naked. Feel free to find a way to justify their penchant for skimpy outfits.

But when The Red Prince wears scale mail, he looks like he's taking armour seriously. He looks not far different from the illustration in the header image of this website. Take the same armour, put it on Sebille, and she essentially gets a couple of pauldrons and super-skimpy boob covering. It's absurd.