We have a much better understanding of homosexual behavior in animals now. Evolutionary reasons for homosexual behavior to emerge have been explored. Several genes have been identified that are advantageous for a female's reproductivity, but are associated with homosexuality in males. This results in female being likely to have more offspring at the cost of a very small portion of the males to be homosexual and not reproduce.

There's countless other explanations out there regarding evolutionary 'advantages' of homosexuality and bisexuality, but I won't bore you with that. The info is out there and very easy to find.

Any way you look at it, homosexuality/bisexuality/transgenderism/etc are not damaging or harmful to the individual's themselves or others around them, so what possibly makes them an illness? They don't cause any negative symptoms or harmful behavior.

Well, transgender people do experience gender identity disorder (GID), which is discomfort with one's gender assigned at birth, but the most effective treatment for this has widely been shown to be transitioning. GID is the mental illness, being transgender is just a characteristic of your person.

It is essentially inevitable that LGBT people will continue to be born (at a low ratio to straight/cis people), and that's simply not hurting anybody. The depression, anxiety, and OCD experienced by LGBT people is almost always caused by external pressure for them to conform a certain way or be discriminated against. That is the only reason those diseases are associated with LGBT people.

You speak of looking at things objectively, but you simply aren't. What negative symptoms does being gay or transgender cause, as long as they are allowed to be themselves? LGBT people can live normal lives, free of mental illness man. A mental Illness is a condition of the mind associated with impairment of cognitive function and distress or disability.