Homosexuality was removed from the diagnostic manuals not do to science, but because a homosexual deliberately infiltrated the psycological association with the deliberate motive to remove it. There is no scientific reason for homosexuality to not be a listed as a mental illness, only political ones.
It honestly depends heavily on the line one draws between "mere personality dysfunction" and "outright mental illness". Pathologizing anger issues
as a whole, for instance, would be a degenerated state of psychology. Similarly, sexuality is such an obscene tangled mess,
moreso in nature, that calling homosexuality a
mental illness is spotty.
Huge swaths of insects are bisexual simply because they lack the
ability to distinguish, and there's not much evolutionary pressure to change it since the sperm can quite easily end up in a female the sire never encountered. We see a few birds that have homosexual couples that differ literally solely in being a male/male pair,
sometimes stealing eggs that are then raised with no discernable difference from normal.
To say nothing of elephants raping rhinos to death, otters raping young seals since they happen to be the right size, dolphin rape-gangs... And that's before touching on prisons and naval duty getting so omnipresent the latter was literally an exception to sodomy laws in the UK because of how unavoidable it proved and the former's a major stereotype to this day!
...TL;DR, we have too many examples of "Any Hole's a Goal" to meaningfully declare male/male intercourse any kind of true
illness since there's quite a number of well-known
categories of its occurrence in both humanity and the wider natural world.
Female/Female's a lot weirder and quite more firmly traced to political bullshittery given the
open and self-described "political lesbians", to say nothing of the exceptional toxicity preliminary data shows before getting canned, but the amount of issues that come with trying to police "toxic" relationships is... Rather dramatic. You can't do that without an
exceedingly totalitarian state.