On the topic of homosexuality and evolution, it is a difficult thing to talk about because we, as humans, behave in different ways when compared to our ancestors (other species). You can not say that "Species A has had homosexual relations as part of their society for X amount of years" and make any correlation to humans because, even though we have evolved from them (key word being 'evolved'), we are able to rationalize things on a much higher level than they can. Evolution boils down to what benefits the entire species, not the individual and for humans, homosexuality does not benefit the (human) species in any form.
Just my two cents.
Let's bring bonobo again into the equation, shall we?
We are very closely related to bonobo:
the bonobo shares more than 98 percent of our genetic profile......................
Not too long ago the savanna baboon was regarded as the best living model of the human ancestor. That primate is adapted to the kinds of ecological conditions that prehumans may have faced after descending from the trees. But in the late 1970s, chimpanzees, which are much more closely related to humans, became the model of choice. Traits that are observed in chimpanzees--including cooperative hunting, food sharing, tool use, power politics and primitive warfare--were absent or not as developed in baboons. In the laboratory the apes have been able to learn sign language and to recognize themselves in a mirror, a sign of self-awareness not yet demonstrated in monkeys.
Although selecting the chimpanzee as the touchstone of hominid evolution represented a great improvement, at least one aspect of the former model did not need to be revised: male superiority remained the natural state of affairs. In both baboons and chimpanzees, males are conspicuously dominant over females; they reign supremely and often brutally. It is highly unusual for a fully grown male chimpanzee to be dominated by any female.
Enter the bonobo. Despite their common name--the pygmy chimpanzee--bonobos cannot be distinguished from the chimpanzee by size. Adult males of the smallest subspecies of chimpanzee weigh some 43 kilograms (95 pounds) and females 33 kilograms (73 pounds), about the same as bonobos. Although female bonobos are much smaller than the males, they seem to rule.
So it would seem that we are related to bonobo (almost) as much as we are related to chimpanzees.
Only, our aggressive, male dominated society seems to descend from chimpanzees, rather than from bonobo.
The bonobo are entirely different: "Amicable, Amorous and Run by Females", as the article describes them.
An extremely important part of the bonobo society is their sexual behaviour:
Nature's raucous bestiary rarely serves up good role models for human behavior, unless you happen to work on the trading floor of the New York Stock Exchange. But there is one creature that stands out from the chest-thumping masses as an example of amicability, sensitivity and, well, humaneness: a little-known ape called the bonobo, or, less accurately, the pygmy chimpanzee. Before bonobos can be fully appreciated, however, two human prejudices must be overcome. The first is, fellows, the female bonobo is the dominant sex, though the dominance is so mild and unobnoxious that some researchers view bonobo society as a matter of "co-dominance," or equality between the sexes. Fancy that.
The second hurdle is human squeamishness about what in the 80s were called PDAs, or public displays of affection, in this case very graphic ones. Bonobos lubricate the gears of social harmony with sex, in all possible permutations and combinations: males with females, males with males, females with females, and even infants with adults. The sexual acts include intercourse, genital-to-genital rubbing, oral sex, mutual masturbation and even a practice that people once thought they had a patent on: French kissing.
Bonobos use sex to appease, to bond, to make up after a fight, to ease tensions, to cement alliances. Humans generally wait until after a nice meal to make love; bonobos do it beorehand, to alleviate the stress and competitiveness often seen among animals when they encounter a source of food.
Lest this all sound like a nonstop Caligulean orgy, Dr. Frans de Waal, a primatologist at Emory University in Atlanta who is the author of "Bonobo: The Forgotten Ape," emphasizes otherwise. "Sex is there, it's pervasive, it's critical, and bonobo society would collapse without it," he said in an interview. "But it's not what people think it is. It's not driven by orgasm or seeking release. Nor is it often reproductively driven. Sex for a bonobo is casual, it's quick and once you're used to watching it, it begins to look like any other social interaction." The new book, with photographs by Frans Lanting, will be published in May by the University of California Press. In "Bonobo," de Waal draws upon his own research as well as that of many other primatologists to sketch a portrait of a species much less familiar to most people than are the other great apes -- the gorilla, the orangutan and the so-called common chimpanzee. The bonobo, found in the dense equatorial rain forests of Zaire, was not officially discovered until 1929, long after the other apes had been described in the scientific literature.
So an entirely different model of sexual behaviour can benefit an entire species.
If we behaved like bonobo, we would live in a much happier society: no violence, no war, no humans exploiting other humans...