Recent studies in animal sexuality serve to dispel two common myths: that sex is exclusively about reproduction and that homosexuality is an unnatural sexual preference. It now appears that sex is also about recreation as it frequently occurs out of the mating season. And same-sex copulation and bonding are common in hundreds of species, from bonobo apes to gulls.
Moreover, homosexual couples in the Animal Kingdom are prone to behaviors commonly - and erroneously - attributed only to heterosexuals. The New York Times reported in its February 7, 2004 issue about a couple of gay penguins who are desperately and recurrently seeking to incubate eggs together.
In the same article ("Love that Dare not Squeak its Name"), Bruce Bagemihl, author of the groundbreaking "Biological Exuberance: Animal Homosexuality and Natural Diversity", defines homosexuality as "any of these behaviors between members of the same sex: long-term bonding, sexual contact, courtship displays or the rearing of young."
Still, that a certain behavior occurs in nature (is "natural") does not render it moral. Infanticide, patricide, suicide, gender bias, and substance abuse - are all to be found in various animal species. It is futile to argue for homosexuality or against it based on zoological observations. Ethics is about surpassing nature - not about emulating it.
The more perplexing question remains: what are the evolutionary and biological advantages of recreational sex and homosexuality? Surely, both entail the waste of scarce resources.
Convoluted explanations, such as the one proffered by Marlene Zuk (homosexuals contribute to the gene pool by nurturing and raising young relatives) defy common sense, experience, and the calculus of evolution. There are no field studies that show conclusively or even indicate that homosexuals tend to raise and nurture their younger relatives more that straights do.

Moreover, the arithmetic of genetics would rule out such a stratagem. If the aim of life is to pass on one's genes from one generation to the next, the homosexual would have been far better off raising his own children (who carry forward half his DNA) - rather than his nephew or niece (with whom he shares merely one quarter of his genetic material.)
What is more, though genetically-predisposed, homosexuality may be partly acquired, the outcome of environment and nurture, rather than nature.
An oft-overlooked fact is that recreational sex and homosexuality have one thing in common: they do not lead to reproduction. Homosexuality may, therefore, be a form of pleasurable sexual play. It may also enhance same-sex bonding and train the young to form cohesive, purposeful groups (the army and the boarding school come to mind).
Furthermore, homosexuality amounts to the culling of 10-15% of the gene pool in each generation. The genetic material of the homosexual is not propagated and is effectively excluded from the big roulette of life. Growers - of anything from cereals to cattle - similarly use random culling to improve their stock. As mathematical models show, such repeated mass removal of DNA from the common brew seems to optimize the species and increase its resilience and efficiency.
It is ironic to realize that homosexuality and other forms of non-reproductive, pleasure-seeking sex may be key evolutionary mechanisms and integral drivers of population dynamics. Reproduction is but one goal among many, equally important, end results. Heterosexuality is but one strategy among a few optimal solutions. Studying biology may yet lead to greater tolerance for the vast repertory of human sexual foibles, preferences, and predilections. Back to nature, in this case, may be forward to civilization.
Suggested Literature
Bagemihl, Bruce - "Biological Exuberance: Animal Homosexuality and Natural Diversity" - St. Martin's Press, 1999
De-Waal, Frans and Lanting, Frans - "Bonobo: The Forgotten Ape" - University of California Press, 1997
De Waal, Frans - "Bonobo Sex and Society" - March 1995 issue of Scientific American, pp. 82-88
Trivers, Robert - Natural Selection and Social Theory: Selected Papers - Oxford University Press, 2002
Zuk, Marlene - "Sexual Selections: What We Can and Can't Learn About Sex From Animals" - University of California Press, 2002