On 26 Nov 2012
Why the hell is gay sex immoral?,
Back to the original question.
"Why is homosexuality immoral?"
I think the question should be two parts:
IS homosexuality immoral?
Why do you think so?
With further subcategories of
What is the morality of the natural world ('self-evident' morality, rights, etc)?
to subcategorical questions, "What is self-evident good and self-evident evil in the natural universe?"
If you really want to piss them off: "How does God decide what is right and wrong?"
Personally, I think there is one best way to define "evil": "Any action taken based on unquestioned belief".
Ergo, any religion that discourages questions would be considered evil.
In the long run of the universe, I go with Schroedinger's "life as anti-entropy" to define what is good (moral) in the natural world: anything that contributes more usefulness to the future than it consumes in resources (hence, anti-entropy: localized thermodynamically, of course).
Now, in the context of any human actions, we can see that any one particular action may be good (useful) or bad (consumptive) in a limited context, but overall, the species itself is basically acting consumptively at this point in time. We know that it didn't always act thus because the species evolved in a natural environment, where it would have had to contribute something (if only extra offspring) to be able to continue to survive over generations in a high-risk, symbiotic environment.
Morality in the 'civilized' sense (intentionally created and legally written) rests within an artificial environment.
Anything goes in this imaginary context because the whole point to civilization is to isolate humans FROM the natural world and its tooth/claw risk-based mutation selection process (morality of those fitted to the environment).
Religious morality is basically a Johnny-come-lately idea based on some past experiences that worked in a human-dominated environment, and has evolved liberally to become blind faith in gods, gurus, government and guns (Might is Right). The latest version is the Invisible Hand Job: a philosophy that allows people to run Open-loop until the environment crashes the resource flow. Somewhere in the near future, humanity will either collapse upon itself to some extent or (less likely) step up to the Responsibility plate and truly become intentional and useful to the universe in a generous way, rather than 'living on Mom nature's handouts'.
In the context of diversity of the species and a robust distribution of variants, I don't think one variation of some humans (homosexuality) is really going to be all that much of a problem either way.