what does 'fixed dichotomies' mean?

In reference to anthropology, it means cultural conceptions based upon two and only two alternatives.

Black & White form a dichotomy, but this is only fixed in reference to physics meaning reflecting all colors or reflection no color. It has no meaning (except psychologically or emotionally) when it comes to skin coloration. The Black & White dichotomy does not allow for Gray (or any other mix of skin tones).

Male & Female form a dichotomy, but XY and XX are not the only possible choices when dealing with living individuals. The "Fixed dichotomy" is in the mind, not in reality.

Perhaps one of the few "Fixed dichotomies" in anthropology is Living & Dead. Here there is no in-between. The same could be said of cultures as being Living or Extinct. There is no middle ground.

The question of Literate & Illiterate as a ""Fixed dichotomy" might be questionable, as not all members of any culture are Literate.

All political dichotomies are relative. Conservative & Liberal are seen as the extreme ends of a very wide spectrum of choices.

Good & Evil are often spoken of as a "Fixed dichotomy," but is any human all Good or all Evil? Most are not either/or but both to some degree.

Either/or classifications.

Binary oppositions.

Masculine/feminine.

Liberal/conservative.