And not thus a disease.
Here's a clue for you: mutations aren't always bad for the organism. Sometimes they're actually improvements.
Not that homosexuality is an improvement, mind you. It's just that declaring all mutations disease is wrong.
That is arguable if you put a lot of value in having nicely decorated rooms and designer shoes.
Wouldn't interpreting homosexuality as a beneficial mutation tend to negatively affect the probability of progeny? An interesting mutation if true!