This is actually going to retcon a few things I’ve said about gender, but eh. I like it better this way. (not a language post really, but I felt it was needed to round out the culture.)
The Sumii people generally recognize three genders, but it’s not unheard of for someone’s identity to fall outside this gender trinary. Two of them are close to what we’d recognize as “male” (tema) and “female” (temi). The third, temen, is basically just “third gender” (in fact, the very word comes from the words for “person” and “three.”) It is not an agender or polygender identity. They normally use the het pronoun, and are treated as a sort of gender neutral category.
Sumii culture doesn’t really have gender roles, aside from some items of clothing being worn normally by men or women (not as a strict rule however, and temen people do not follow any sort of clothing restriction) and the Tsarehi almost always being female (again not always the case – in some villages it is considered an archaic rule, and you see men or temen in the role, and others consider temen the only ones fit to be Tsarehi).
Gender is not assigned at birth. All children are considered non-gendered until they’re able to grasp the concept, at which point they’re allowed to choose what fits best for them. However, this doesn’t mean people don’t get an idea in their heads what gender the child is – some do end up calling a child “little sister/brother” or giving them a more gendered name or pronouns based on their anatomy. And speaking of anatomy… I’m not going to go into too much detail at the moment what their genitalia is like, but there is, for lack of better terms, one that is considered “male” and one that is “female,” and many that we would call “intersex.” Intersex people are much more common here than they are among humans. Most end up falling into the temen category, but some do identify as men or women.
As for sexuality, they have the same range as we do – people who love different genders, the same gender, multiple genders, or aren’t interested in anyone. There are no specific words for each one, and in fact they aren’t considered separate identities at all. For the Sumii, love really is just love. Polyamory is also pretty common. In fact, you’d probably just as often see a family with three parents as you would two!