>>6343431 is a mental illness because it is a PARAPHILIA.
It is an atypical preference which is contrary to the natural necessity of creating offspring which is the reason EVERY animal exists, to propagate more of it's own species. Homosexuality is antithetical to this.
Anything mental that inhibits reproducing is an illness. By definition reproducing is the only reason life exists in the first place.
Moreover, 'homosexual behavior' in the animal kingdom is an expression of DOMINANCE and they often find female mates or else die as an evolutionary accident.
Very rarely is animal homosexuality anything but an expression of dominance, they do not treat each other as if they were mates.
1,500 animal species is hardly EVERY single last species as you try to claim. This is just sophistry.
Also dogs and other animals do have mental illnesses and it is very much possible.
Also nobody here claims depression is fake, but a lot of people who claim to have depression don't really experience it. This is just a strawman.
>P-people only dislike gays because of religion! 
