So after encountering an unrelated trash story that involved aliens that were stated to be genderless electing to fall into our exact gender binary and heterosexuality despite having no reason to, I’ve decided to lay out my Starlights gender headcanons.

Under a cut because I know that people have strong Starlights feelings.

Firstly, I head canon that gender on Kinmoku works differently on earth, in that there isn’t any tie to biology (I think that the limited shape-shifting abilities we see in the anime are a trait of all Kinmoku-ians, rather than just senshi). I figure each of Kinmoku’s senshi are representative of different genders (I also headcanon that there are at least two other Starlights who died in Galaxia’s take over, but that’s not entirely relevant), and the Princess has no gender, because her role as ruler takes precedence over any other role she might take.

So when the Starlights come to earth, a quick look at the general culture tells them they’ll have an easier time posing as men, so that’s what they do.

Taiki. however, is interested in figuring out the real way they fit into earth’s gender structures (I think a part of her she wouldn’t acknowledge is thinking of the possibility that they’ll never reclaim their planet, but maybe they really could save this one and live here with Kakyuu). She does a lot of research, and after being around humans firsthand enough to get a sense of things, she settles on demigirl. It fits her to a T, she feels like an earth woman to an extent, but also not, so that’s what she tells Seiya and Yaten (and, eventually, earth’s senshi, starting with Ami and Mako).

Taiki tells Yaten that they might also consider demigirl, but Yaten does not care. This is earth, not home, it’s never going to be their home, there’s no point in getting attached to the culture just like there was no point on any of the other planets they stopped at. Even when it’s clear that they’ll be staying awhile, and even after Yaten starts feeling attachment to certain aspects of earth, they do not care about gender. Except boys are gross. Yaten is nothing like the sweaty lumps that are highschool boys, it’s practically an insult to group them together. Yaten is very emphatically Not A Boy. But then they get annoyed when people assume that makes them a girl. So they end up insisting they’re agender, despite Not Caring At All. 

Seiya starts out with a very simplistic view, as Seiya doesn’t listen to Taiki’s gender information at all. Seiya sees what boys wear and how they act, it matches pretty well with how Seiya likes to dress and act, therefore, Seiya must be a boy on earth. And it’s this view that leads Seiya to think Haruka’s a man in episode 180 (the concert episode). She dresses similarly to men and dates a woman, surely that’s what makes a man? So Seiya actually keeps referring to Haruka as a man for quite awhile, until she’s finally like, “Now I hate you anyway, but I swear if you call me a man one more time, I will personally ensure you never come to earth again.” And Seiya realizes it’s not their normal antagonism, Haruka’s actually upset on a personal level, and Seiya has no idea why. So Haruka has to explain through gritted teeth what a butch lesbian is, and then back up and explain lesbianism, because Seiya didn’t see any representation of it in day to day earth life and media. 
Then everything sort of clicks in Seiya’s head and she’s like, “Oh, that’s what I am.”
And Haruka’s like, “What no, you’ve been saying you’re a boy all this time you can’t just copy me.”
“What, are you afraid I’m going to steal your girlfriend?”
And even though Haruka is deeply offended by the suggestion (and asks Michiru to assure her that she’d absolutely never ever date Seiya Kou when she goes home that night), the whole thing actually sparks a tiny amount of affection in her for Seiya, since Seiya’s confusion about herself hits on Haruka’s compassionate dork side.