A report that lesbian pairs are common in a Hawaiian Albatross colony (Young et al, Zuk & Bailey) is the latest finding on animal homosexuality to raise some media attention (Daily Telegraph, Wired, Times Online). Many people are afraid to find out that homosexuality exists in animals, and therefore is "natural," because what is natural is often deemed morally acceptable.
David Hume, making perhaps the most important point in the history of ethics, stated over 200 years ago that we should not argue about how the world ought to be based on how the world is. Yet research on animal homosexuality still brings people on the verge of this error, by triggering the question: is it "natural"?
Hume himself did not speak enthusiastically of the "shameful and unnatural lusts […] which, by our law, […] justly expose the offender to be punished by death" (Commentaries on the Law of Scotland). He excused, however, the "Greek loves" as arising "from a very innocent cause, the frequency of the gymnastic excercises" (An Enquiry Concerning the Principles of Morals, Schmidt's summary of Hume's moral ideas).
Hume thus did not follow his own advice against confusing what is natural and what is moral, nor do many people today. The "unnaturalness" of homosexuality should not figure in discussions of homosexuality and human society. Lesbian Albatrossess are an interesting biological phenomenon, but should not burden us with moral dilemmas.
And from my neck so free
The Albatross fell off, and sank
Like lead into the sea.