Maybe you could use your current gender, but for some people they want to include that variability in their identity. Just because they are also male/female/whatever doesn't change that they have identified as something else in the past, and they want to include that in their current self. Is all attraction about looks? There are men who look feminine and women who look masculine and intersex people who choose to dress one way or another. And it's not true that the only two visible genders are male and female. There are trans women who keep their beards, people who dress entirely androgynously, there are cis men who like to wear pretty dresses but still identify as male. I think saying 'all people conform to these two things' is dismissing all the people that do. It may be a majority, but not all. I tend to think of it like this (I am assuming a couple of things about you, but they are likely to be true, also sorry, this is a bit gross): Image having sex with a puppy. You may love puppies and think they are cute and you want one as a friend, but that is still revolting and not something you would ever willingly do. I wouldn't say 'oh you just have a low libido', its that you just don't have any reason to. You could argue that you can have low libido to anything because it is just a lack of desire, but then maybe there should be a descriptor for something totally outside that scale. So you think people shouldn't try and find others like them for support? Being part of one society does not prevent someone from being a part of a different one too. I do think that very insular societies can be unhealthy, but that doesn't mean that groups of people with certain identities or issues can't have a group around them. Sorry, perhaps I phrased poorly. There were large groups of colonialists that went to the Americas due to religious persecution, Quakers and Puritans and so on. Treating their perceived identity as mental illness and something you want to make them fix is the first way you make them feel rejected. Yes it is odd, but is it harmful to others? If the biggest issue with how someone wants to be is that others hurt them for it, then are they the ones that should be made to change? I'm not saying that thinking you are literally a dog/cat/wolf/space whale isn't potentially an issue, but I do think that making them feel safe and welcomed is the first way to help them. Also side note - not all beast/whatever-kin actually literally think that they are an animal/other being. For a fair few of them it is an equivalent of belief in reincarnation (they have been an X in a past life), which is usually not treated as mental illness. When you get to that point then you start having to argue with religion, which right now I'd rather not do.
I have to say, I disagree with your first point. Why DO people use something that happened once or twice a long time ago in their identity? It does not change the fact that right now they are what they are. As for attraction - no, standard attraction is not. However I have never seen or heard of anyone being genuinely, non-jokingly aroused by something not physical. Voices? Yes. Body parts? Yes. Certain actions done by people? Yes. But I've never heard of anyone being aroused because of someone else liking something mundane, or believing in something. As for the two visible genders, there's a little tidbit: trans women keep a male attribute (and to even maintain a beard, they most likely need masculine reproductive hormones, which tend to make them keep a masculine facial structure). Dressing androgynously does not mean looking androgynously - the person can have masculine or feminine features (and if they don't, I think there was a research that made people see what they want to see - so people who are into women will think it looks ever so slightly feminine, and people who are into men see him as slightly more masculine). Cis men wearing dresses (isn't that a fetish?) is a combination of MALE physical attributes with doing something (which is covered above). It is not conforming to those things - it is just that because we are human we have human features - and there is no true 'genderless' feature in human biology. As for the sex thing - it actually supports my opinion. You're not supposed, biologically, to reproduce with a puppy. However, you are supposed to mate with someone you find attractive, biologically - and not feeling it is either mental or genetic illness. So, in my opinion, either something is wrong with them or it IS low libido. And for TiffanyAching's comment, there's another issue - family love is completely unlike romantic love. Family love is closer to being the ultimate friendship bond - a 'bond of blood'. I know three ways to acquire such a bond, and two of them involve some form of heavy trauma. Also, family love simply means you appreciate family. Usually, family love does not make you giddy or overwhelmingly happy just by thinking about them. You don't feel the need to constantly iterate your appreciation and affection to them. The Greek even had different names for them - despite their distortion, philia used to mean this family or friend bond (when it was not storge - the bond of luck, where people have lived significant things together to the point of being able to bond over it). By comparison, the romantic love was Eros - which C. S. Lewis brilliantly explained as "the difference between wanting a woman and wanting A woman". It is not necessarily sexual in nature, but usually it either fades out or helps develop it. I am all for people trying to get support for others like them. The issue, here, is that they are being VERY exclusive, it's not "I want to get support from people" but "I don't want to meddle with humans". They're excluding sympathizers. And that is the biggest issue. As you said, being insular is unhealthy - and so far, I have not seen any inclusive or even open-minded people of that group. Also, these religious folks, if I remember, were also not very welcomed to the new lands BECAUSE of these beliefs. They mostly kept to themselves and even to this day their population is insignificant specifically because they rejected anything that did not fit their point of view. As for the 'treatment' - I mean it in the best of way. It's a shame that mental illness is to be ashamed of - but admitting there's a problem is the first step to getting better. And as far as being harmful - I would say yes, because these people also tend to be memetic - far from all of their members start out as feeling this way, but only develop these symptoms after prolonged exposure (which might be enabled/accelerated/simplified because of prior mental issues, like heavy rejection from their peers). As for the reason why I want to see them as being in need of help is not because they are being hurt for it - assholes are going to find any reason to hurt people, they are just an easy target (mostly because either they're insecure to the point of fighting about it, like a lot of high-schoolers, or because it would be like 'fighting the sapienarchy' for them). And for the 'feeling safe' part, I DO agree. But safe does not mean comfortable. When things are comfortable, humans feel like everything is okay - and it is not. There is no way for people with mental issues to comfortably heal themselves (yet). Comfort, for a lot of mental illnesses, is a bit like a resonating chamber. "I am okay the way I am, therefore I will keep doing it". And for the side note: I disagree, but only because of anecdotes (so take that as you will). But so far, everyone that went as far as calling themselves X-kin (or, a few year back, furry/scalie for the folks specifically relating to furred animals and reptiles) did. Those who do not usually see it as "I am human, but X is my spirit animal/what my soul comes from" or something similar. And they are not treated as mental illness (and mostly shouldn't) specifically for the reasons you said - it is more of a spiritual belief than an identity.
Actually a think the puppy analogy was a bad one. I use it for myself to understand what is like to have absolutely no interest in something, but there's a lot of consent issues around it. I guess it's more like how you would think of family. People are evolutionary and socially conditioned to have no sexual desire towards family members, and it's still a 2 person consensual and (usually) loving relationship.