Why they really bothers me though, is that they are never biologically, or even logically, consistent. First off, ask yourself: why does this magic differentiate between men and women? Does it do so on the basis of sex, or gender?
This should be obvious to most people with even the barest education in history, anthropology, sociology, or psychology, but gender is the performance of a social role (often based on biological sex, though not always). It is psychologically and sociologically constructed. Many societies have had - and still have - third genders, or the recognition of "two spirited" individuals, or bestow upon people the ability to change their gender if they wish (e.g. sworn virgins). If magic is gender-based:
-Societies with more than two genders would have more than two types of magic
-There would be individuals of one sex that used the other gender's magic
-There would be individuals who could use both types of magic
Yet in fantasy worlds with gender-based magic systems (i.e. magic chooses individuals on the basis of their "spirit" or masculine/feminine "soul" or what have you) almost never seem to have these. (One possible and surprising exception to this was Dark Souls, actually, where one biologically male character who dressed and lived as a woman also used "female" magic.)
If an individual's ability to use magic is purely based on biology, how does it make this distinction? Is it primary or secondary sexual development, or is it genetic? If it's sexual development, what magic do intersex persons with ambiguous genitals get? If it's genetic (XX vs. XY chromosomes), what magic do persons with XXY, XYY, X, XXX, etc., chromosomes get? Would some XX individuals with all of the primary and secondary sex characteristics of males not then use "female" magic, too? And certain XY individuals with the primary and secondary characteristics of females not use "male" magic? Lacking our knowledge of genetics, how would your fantasy society rationalize the existence of these people? The whole notion of sex-based magic falls apart completely based on what we now know about the biological basis of sex. If you were to have sex-based magic:
-There would either have to be more than two types of magic (possibly a great many) or intersex individuals would have to have access to both
-There would be individuals who to all observation appeared to be one sex (including that sex's genitals) who used the other sex's magic
Which would be an interesting notion were someone to actually attempt it. Perhaps your imaginary society of magic users feels that infertility is the "price" certain "special" women (i.e. individuals with androgen insensitivity syndrome) have to pay for access to "male" magic; sterility is the price for men (i.e. XX males) to use female magic. Or that certain physical abnormalities that often accompany chromosomal abnormalities are the cost of accessing special/mixed magic. I've never seen anyone try their hand at this either.
No, no, instead it's always same old, same old, saidin/saidar, sex = gender, intersex and transgendered people don't exist in this world. And the arguments I always hear against my proposals are this:
-Well intersex/transgendered people are such a tiny minority nobody cares about them and I don't need to bother thinking about what happens to them!
...Don't you? Complete androgen insensitivity syndrome occurs in one in 20,000 births; there's at least one beautiful woman in each fair-sized fantasy city who can use man-magic - 12 of them in fantasy!Rome alone. You don't think your hero has ever heard stories about that?
-The magic "knows" their "real" sex/gender!
...How? That's very progressive of you in the case of transgendered persons, but it still means you've got people who appear to be one thing with the power of another. You're cool with that? Awesome! You can be as cool as the Dark Souls team. ...Now what are you going to do with people who truly don't identify as a gender or identify as both?
-Magic is bestowed by a being/sentience who does so only to individuals with unambiguous genitalia who identify solely as the gender ascribed to their sex - hah!
Your being is kind of dick. Why does it do this? "To make the author's life easier" does not make for a compelling or logically consistent world. What does it do about societies with a third gender, anyway?
-There are no societies with a third gender, there are no transgendered or intersex individuals, and no persons with chromosomal abnormalities in my world - hah!
So... you're writing about a non-human fantasy race? That's cool. Just make it clear from the outset that your fantasy people in no way resemble real people. Most fantasy authors call their wish-fulfillment race "elves" but you could always try to come with something more creative.
-Shut up shut up you feminist cow. Stop trying to police my imagination!
This is by far the most common response I receive to the above whenever I talk about the above with someone who wants to make a gendered magic system. How is describing human biological and psychological realities "policing" anyone's imagination? It's fantasy; if you want to make a magic race with no transgendered or intersex individuals, feel free - just don't pretend that's "us" in any way, or that it is somehow an accurate allegory for the "differences" between the "sexes". (Which they inevitably do.) Wouldn't it be more creative to take human variation into account and magic more than two types of "gendered" magic? I certainly think so.
It's magic - it's mysterious and unknowable and I don't have to explain it. Hah!
...But you've already given your magic "rules" that it operates by; those "rules" are incomplete or nonsensical. Say that you've establish that right-handed people get fire magic, and left-handed people get water magic. Someone comes up to you and asks "what do ambidextrous people get?". "Nyah nyah I don't have to tell you it doesn't matter magic works in mysterious ways" would be the cop-out of the century, would it not? And "ambidextrous people are a tiny minority so they don't count/magic knows their "real" handedness/ambidextrous people don't exist in my world" would make you look like an unimaginative douchehat with serious issues, no?