Conservative author James Dobson, noted for his strong stance against what he calls "the gay rights agenda," once asked, rhetorically, why it is that, with thousands of years of animal domestication, and the great increase in modern times of scientists in the field, and wildlife photography, male-on-male sexual penetration had never been documented? Conversely, Bruce Bagemihl, author of _Biological Exuberance_, documented extensive and varied occurrences of homosexual behavior in animals - including male-on-male penetration in three species: the American bison (Bison bison), the bighorn sheep (Ovis canadensis), and the orangutan (Pongo pygmaeus). His coverage of bighorn sheep was particularly revealing, as he documented, first, the reluctance of the homophobic biologist to reveal his findings, the same scientist's subsequent realization that, yes, he really was seeing what it looked like he was seeing, and finally, the pervasiveness of male homosexuality in the bighorn sheep. As Bagemihl points out, bighorn rams mate with ewes only during a brief, defined mating season, but outside that mating season, they constantly mate with other rams.
In the human homosexual community, there is a term: heteronomative. It means that because heterosexual humans are by far the majority (approximately 90%), their ways of thinking are defined as "normal." Thus, even in biology, there is an assumption that heterosexuality is normal for animals, and when an animal exhibits homosexual behavior, this is an exception to the norm which must be specifically explained. Classic Darwinian theory does tend to support this, with its emphasis on reproduction as the end goal. (Likewise, many religious conservatives also consider reproduction as the sole purpose for sex in the marriage.)
The problem with heteronormative explanations for homosexual behavior in animals is that they discount a fact of human biology. Our emotions, including love, are controlled by the same chemical compounds in the brain as those of animals. Serotonin, for example, causes behaviors in animals which, in humans, would indicate a particular mood. Serotonin in humans causes that same mood. In our zeal to avoid anthropomorphizing, we too easily discount this fact. If animals and humans have the same processes of brain chemistry, it is quite reasonable to say they experience the same moods and feelings.
Thus, when we see a pair of swans, for example, forming a pair bond prior to nesting, we can show, by measuring concentrations of chemical compounds in the brain, that this is physiologically the same as what happens in the brains of a human man and woman falling in love. It is doubtful that the swans are aware of Darwinian theories about why they form a pair bond; more likely, they "feel" the pair bond forming, and act accordingly.
Now, it sometimes happens that two male swans will form a pair bond and build a nest together. Birds are anatomically different from mammals, so sex works differently in them; but male-male swan pairs are observed having sex in just the same manner as male-female pairs. Of course, these matings do not produce eggs; but these male-male pairs are also known to "adopt" eggs from nearby nests, incubate them, and raise them. In some cases, these nests successfully fledge more cygnets than do "normal" male-female nests. So in the greater scheme of things, homosexuality in the swans can be beneficial to the species as a whole.
There are various animal species with cooperative breeding systems, that is, in which one pair will produce all the offspring, and other individuals, called "helpers," assist in caring for the young. Somehow, the genes for "helper" behavior still get passed on, even though the "helpers" may never reproduce themselves. Perhaps this is done through recessive genes - genes which only manifest when the individual gets them from both parents. If only one parent contributes the recessive gene, no outward sign of that gene appears in the offspring. In humans, a tragic example of this is sickle cell disease: a child who gets one copy of the recessive gene will live, and be resistant to malaria, but a child who gets two copies will die. Enough get only one copy, the gene remains in the population. Although there is as yet no proof of a "gay gene," either in humans or any animals, if one exists, it is likely passed on in the same way, recessively.
It is difficult to draw exact parallels between animal societies, and human society. Different animal species have different mating systems, so there is not much point in saying because species X does or does not do Y, that humans should do likewise. Nevertheless, there is much evidence in nature that homosexual behavior is a natural part of the world, and it stands to reason, then, that it will also manifest in some manner in the human species.