Some misconceptions that need to be cleared up...
Are the Sailor Starlights men or women?
The Sailor Starlights are female, period. Their civilian alter-egos, the Three Lights, however . . . That's a different story.
Let's set this straight once and for all: In the manga, the Three Lights are women disguised as men. In the anime, the Three Lights really are male while on Earth. They revert back to their female forms when they transform into the Sailor Starlights.
In the anime, it is suggested that the Three Lights are normally female on their home planet of Kinmoku, and only became males while on Earth. The first hint is in Seiya's prolouge: "We must search quickly for our Princess and protect the galaxy from the evil hand of Galaxia . . . To that end, we've become idols, we've become high school students, and we've become boys." This would suggest that Seiya was not a boy previously. Also, in one of the final episodes of the anime, Kakyuu asks Seiya "Why did you choose... these... forms?" seeming to imply that she is not used to her guardians as being male.
Sarah contributed this comment:
In episode 194, Seiya is standing at Usagi-chan's desk with a rose, Seiya that it's pretty much the end of being Seiya Kou... in episode 195, he tells Usagi that Seiya's time is short... It's like Seiya stopped thinking of himself as Seiya, and resumed being Fighter at that point. Even his attitude changed... Thoughtful Seiya is not something oft seen outside the manga.
I agree. If I recall correctly, the actual line from episode 194 was "This is my last day as Seiya Kou."
In this website, I usually refer to the Three Lights as male (ie, "he") but refer to the Sailor Starlights as female (ie, "she"). The only exception is when I talk specifically about the manga, which isn't often. In that case I would refer to Seiya, Taiki, and Yaten as "she." This is because most of the information on this site is based in the canon of the Stars anime.
Although, it should be stated once and for all that the Starlights are female in their true forms, period.
But then again, the above information all assumes that people from Kinmoku even have genders that we would label as "male" and "female." Rushita has written an excellent essay that deals in depth with the tricky possibilities of alien gender. Please read it!
Why do they all have the same surname, "Kou"?
The Three Lights DON'T have the same surname. Actually, "Kou" is their given name. The Three Lights write their names in Japanese order, in which the surname is stated first. Thus we have Seiya Kou, Taiki Kou, and Yaten Kou. Were we to write these names in the Western order that most of you are more familiar with, however, we would have Kou Seiya, Kou Taiki, and Kou Yaten. For more information (and some debunking evidence for naysayers), please read this page.
Are the Three Lights brothers?
No, they're not related. Again, please refer to this page for more information.
Do the Sailor Starlights DIE in the manga?!
Unfortunately, yes. But their death is not "permanent" - they are resurrected in the end, and return to Kinmoku with their princess. Allow me to explain:
In the anime, all of the Inner and Outer Senshi (except Sailor Moon) die when their starseeds are taken by Galaxia. They are resurrected when Sailor Moon defeats Chaos and releases all of the stolen starseeds. In the anime version, the Sailor Starlights never die and thus never have to be resurrected.
In the manga, all of the Inner Senshi, Outer Senshi, and Sailor Starlights die when their Sailor Crystals are taken. Once again, Sailor Moon defeats Chaos and releases all of the stolen Sailor Crystals. The difference is that although we see the Inner and Outer Senshi come back to life, we never see the Sailor Starlights or Princess Kakyuu again. Because these characters are not shown after their death scenes, many fans believe that they simply died and stayed dead.
HOWEVER, after Sailor Cosmos defeats Chaos, she explicitly states, "The Sailor Crystals of the sailor soldiers scattered in the Cauldron, are returning to their original forms, and going back to their own planets. They are all still just starseeds born from the Cauldron, but as they reach their respective planets, they will grow, and return to their forms of before." We know that this includes *all* of the Sailor Senshi defeated by Galaxia and the Animamates, because earlier in the story Galaxia gathered all the Sailor Crystals together and threw them into the Galaxy Cauldron.
You see, just because a character isn't explicitly shown "rising from the dead" doesn't mean that they were excluded from the miracle that Sailor Moon performed in the final act of the manga.
It's more logical to assume that the Sailor Starlights and Princess Kakyuu were simply sent back to their home planet, Kinmoku, after the battle with Galaxia.
In the manga, is Seiya a lesbian?
In the manga, Seiya (100% female, remember) does indeed fall in love with Usagi, and it is also strongly implied that she has romantic feelings for Princess Kakyuu. But that shouldn't surprise you. There's already a well-established lesbian couple in the Sailor Moon canon: Haruka and Michiru. Naoko Takeuchi apparently has no problem with including same-sex love as a theme in her works. But then again, Seiya's motives in the manga are a bit more complex than they are in the anime, regarding her infatuation with Usagi.
I have more questions!
Are the Sailor Starlights actually princesses, like the other Sailor Senshi? If so, what "planet" are they the princesses of, if any? What was their past like, on Kinmoku? Who were their parents? How did they first meet?
Um, I don't know. The page Mother Planet ought to answer some, but not all, of those questions. Naoko Takeuchi NEVER reveals anything about the past of the Sailor Starlights or their lives on the planet Kinmoku. I know that there are a lot of silly rumors floating around the internet, but most of them come from discreditable sources and should be ignored. There has been fanfiction that has approached these questions, but that's all it is: fanfiction! Never confuse a good fanfic with the canon truth.