Homosexuality in a rational society would be classified as a mental disorder. And up until the 1970s, it was. A genetic explanation can be ruled out on evolutionary grounds.
Yes, homosexuality occasionally happens in animals, but 1) humans are animals too (though we aren't merely animals) and 2) homosexuality isn't normal for animals either.
Secondly, just because someone has a mental disorder doesn't imply they should have no rights-- it only implies homosexuals aren't normal people. If narcissism and things like "conduct disorder" are considered mental illness, then I see no reason to exclude sexual confusion from the list.
I did take a quick look at DU. I noticed they aren't talking about this one yet. Maybe they don't want to know the truth: living a homosexual lifestyle is dangerous to themselves and their children.
So would being a member of the Baptist Church. Then again with all the Baptists I know in therapy trying to figure out the insanity of their parents, I guess it's a MD by default.
A rational society would classify it as a fetish, like feet or leather. So long as the affected person did not suffer or harm others, it would not be classified as a disorder. Instead of this rational approach, however, we've chosen to swing from one extreme to the other -- now homosexuality is an "identity," like Jim McGreevy the "gay American." Had we been more rational, that would've been as remarkable as Joe Schmoe the "foot-sniffing American," Hans Kent, the "American Who Likes to Wear Women's Underwear But Only in the Bedroom," or James Hammel, "the American Who Prefers Blondes."
According to my Merck Veterinary Manual, homosexual behavior in farm animals is a treatable disorder.