Grisha Powers = Sexuality
Guys, I figured it out. Grisha powers are a metaphor for sexuality. Think about it:
Using Grisha powers tires out Grishas, but in the long term they are healthier for it. They become all . . . luminous after using it.
The way the powers are viewed by various countries. In Ravka it is an unfortunate necessity. Fjerda deems it unnatural. The Shu Han are trying to figure out how the hell it works. Kerch literally turns Grishas into prostitutes. Only in Novyi Zem is it seen as a positive, natural thing.
I’m going to say that Etherealki are heteros. See The Darkling and Alina, who are super attracted to each other even though hate it. The Darkling thinks Grisha powers should be used for particular purposes, not for frivolous reasons.
Corporalki are bis. See Nina (often shipped with Inej) and Sergei, a Corporalki that can hook up with a Etherealki, Marie.
Matherealki are gays/lesbians. This is why David shows no interest in Genya (who is bi, but maybe leaning toward lesbian?). Jesper is supposedly interested in girls, but I have never seen any evidence of it. He hides his Grisha power the way that some people hide their sexuality.
Alina is not gay but hides her literal sexuality as well as her metaphorical sexuality (Great job, Mal!). The Darkling at least encourages her to use her power, but only in the way that he wants her to.
The Grisha triology (I’m only halfway through) is about an old man trying to control a young girl’s Grisha power.