We can try to define sexuality (and/or gender) according to our personal opinions, but by definition, one's non-heterosexuality is not a disease.

I'll admit I don't understand why someone would not naturally be heterosexual, but there are significant variations in sexuality in certain species, as noted above, and secondly, if part of the decision about sexuality is a choice sobeit. Is there really any harm in letting people determine their own sexuality (and gender)? (You of course don't have to answer that.)

Mainly, to address this issue, one would first have to consider biology and human rights, and perhaps anthropology. One could thereafter consider the potential disease, which would depend on the initial basic assumptions, and would expand the discussion to the necessity for clinical evidence. For example, obesity is considered a disease today, though it was not considered a disease 500 years ago. Homosexuality was, however, depathologized in the US in 1973 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4695779/). If you and the government want to consider homosexuality a disease or even "gross indecency", there would be the potential return to chemical castration or prison time for anyone prosecuted as a homosexual, as happened to Alan Turing in 1952 (https://en.wikipedia.org/wiki/Alan_Turing). Does that seem appropriate? (You also don't have to answer that)