Homosexuality is not uniquely human. A wide variety of mammals and birds experience same-sex sexual encounters. Williams, Adaptation and Natural Selection, Princeton University Press, 1966, Page 204; Bagemihl, Biological Exuberance: Animal Homosexuality and Natural Diversity, St Martin's Press, New York, 1999. Just as with humans, the spectrum of homosexual behaviors is plentiful.
In some cases, homosexual encounters resemble heterosexual courtship. Males perform to each other the same repertoire of behaviors that they normally display when courting females. At other times, males may simply display affection to each other as observed in male lions who may rub heads and roll around together, without overt sexual content.
Sexual contact, however, is fairly frequently observed between members of the same sex. Female bonobos, a highly social primate, engage in mutual genital rubbing, and other explicitly sexual types of physical contacts. During same-sex grooming, male vampire bats develop erections. After engaging in prolonged sessions of affectionate rubbing, male giraffes display mounting behaviors that culminate in apparent orgasm. The same kinds of behaviors are observed in birds. Of tern pairs who shared in the care and tending of eggs and offspring, it was discovered that 12% were actually female couples. Apparently, females fertilized their eggs with paired males, but raised their offspring with a partner of the same-sex. Lesbian behaviors were also observed in Japanese macaques. Females have been observed to spend extended periods of time together, grooming and participating in sexual activity that ends in orgasm for both partners. For more examples, read Bagemihl, ibid.
Despite widespread documentation of homosexuality in the animal kingdom, evolutionary biologists are confounded by it. As a fruitless, reproductively dead-end, behavior, it logically would be culled from the population because its practice does not result in children and therefore would not be inherited by successive populations. One solution to this dilemma is to say that homosexuality is not inherited, but is a psychological disorder, or even a disease. In fact, homosexuality was once classified as a psychological disorder, but no longer is. More recent evidence suggests that there is a genetic component to homosexual behavior. See, e.g., Hamer and Copeland, The Science of Desire, 1994. Reproductive functions are generally less precise in execution and more imperfectly timed than other behaviors because the selective pressure to maintain them in the gene pool are so great. Williams, Pages 204-205. So what if a little energy could have been more constructively spent.
If homosexuality is not a disease or a maladaptive behavior, then what is it? Many homosexuals do reproduce, and could even be more fecund than heterosexuals when they do, keeping homosexuality flowing through the gene pool. The possibility that homosexuality has reproductive advantages for carriers of the genes (e.g., greater fertility, or better parental care) who do not express the behavior is one explanation for its stability in the gene pool, making up for the fact that the gene stops in individuals who expressly manifest the behavior. It can also be explained as an adaptive behavior to ease stress in social encounters, and to facilitate cooperation between individuals who otherwise possess competing interests.