since it appearent throughout many mammal species I would assume, there is some benefit or at least tolerance of it for evolution in general, so it is obviously taken into account by nature (if you want so). Mantel illness is just the wrong term imo. its just varying from the majority, which does not mean it is an illness. Similar for ADHS for instance. Ppl just suffer from it in our society, because this variation of perception of environment is not accepted by most other ppl.

indeed that is possible and a valid argument, but since we are talking about a complex system here, its not very easy to prove so i can only give arguments and fun facts for some conditions and state personal opinions based on that

basically to diagnose something as an illness from a logical point of view, you dont need the WHO definition, you just need the base for it: any condition that lowers the chance of your genes surviving further is an illness, and this applies ofc to the group level. so heres a few conditions:

- mild sexual perversions: some things are proven to not be an illness, some count as paraphilia though and its hard to distinguish. for example pedophilia is an illness (object, child, is useless for generating offspring so this is an obvious one). liking blowjobs is not an illness because even though you sacrifice some of the genetic material, you more than make up for it because the antigens the woman swallows will make pregnancy more possible when sex does happen. there are very blurred things that count as complex systems and we dont know much about those (for example already mentioned homosexuality), and in this case i just go with intuition as jessi said (people intrisincly classify illness as "weird"), and this is actually a very strong argument as instincts arent random, they formed under generations of evolutionary pressure so they are generally right

- intelligence is not an illness even though its individually maladaptive, as it is extremely important on the group level

- ADHD is an illness as there is absolutely no gain from it on either individual or group level. id say its just a proof that we are getting genetically "stained" due to darwinian conditions removal thanks to the progress of medicine

- autism is an illness, but as its connected to outlier high IQ, on the group level it is a good thing. well, at least the thing that goes with it is a good thing. same goes for example for sickle cell anemia, which is a protective mutation against malaria and if you live in those areas, you really WANT that mutation. so yeah i agree some conditions might have a point to them, id still classify them as illnesses although this is debatable

so the same goes for homosexuality. it might have been important at some point in the evolution (more caretakers for the children), and a strong argument for this is that it mostly appears in animals with a social structure. another strong argument is that lesbians are looked upon with less hate than fags, and this could be due to women being better at caretaking so it would be more "allowed" by the society. so yeah, it might have been needed at some point in human evolution. but its really not anymore. we dont have natural predators and parents alone are more than enough to take care of the child imo. again, its a complex system and i dont claim to know everything, im just saying homosexuality in the age we are now is unlikely to give a net gain on either individual or group level