Homosexuality is regarded as being unnatural, but that's a paradox: nature abounds in homosexual behavior in animals. At least 1,500 species have been observed engaging in this behavior, and in 500 of those it is well documented. This tendency occurs especially in social species, paticularly marine birds and mammals, monkeys and the great apes. For example, male penguin couples have been documented to mate for life, build nests together, and to use a stone as a surrogate egg in nesting and brooding.
In the case of Bonobo, the dwarf chimpanzee, the closest living species to humans, the community model is based on female solidarity and good relationships between the sexes. Sex is at the basis of their social relationships and the weakest conflict is solved through sex, no matter it the other is male or female, as part of social bonding.
It appears clearly now that for many species, like in humans, mating may not have a necessarily reproductive role. And observations made on our closest relatives, monkeys and apes, could explain the homosexual behavior in humans.
Disclaimer: I am not a proponent for homosexuality.
Source: Wikipedia