The naturalness of homosexuality in non-human animals is opposed by conservative religious groups because it points to the naturalness of homosexuality in humans. But it is now clear that copulation could also be used for alliance and protection among animals of the same sex. In situations when a species is mostly bisexual, homosexual relationships allow an animal to join a pack.

In wild animals, researchers will as a rule not be able to map the entire life of an individual, and must infer from frequency of single observations of behaviour. The correct usage of the term homosexual is that an animal exhibits homosexual behaviour, however this article conforms to the usage by modern research applying the term homosexuality to all sexual behaviour (copulation, genital stimulation, mating games and sexual display behaviour) between animals of the same sex. In most instances, it is presumed that the homosexual behaviour is but part of the animals overall sexual behavioural repertoire, making the animal "bisexual" rather than "homosexual" as the terms are commonly understood in humans, but cases of clear homosexual preference and exclusive homosexual pairs are known.

No species has been found in which homosexual behaviour has not been shown to exist, with the exception of species that never have sex at all, such as sea urchins and aphis. Moreover, a part of the animal kingdom is hermaphroditic, truly bisexual. For them, homosexuality is not an issue. "In bonobos for instance, strict heterosexual individuals would not be able to make friends in the flock and thus never be able to breed," Bockman told LiveScience. "In some bird species that bond for life, homosexual pairs raise young. If they are females, a male may fertilize their eggs. If they are males, a solitary female may mate with them and deposit her eggs in their nest."

Some researchers believe it to have its origin in male social organization and social dominance, similar to the dominance traits shown in prison sexuality. Others, particularly Joan Roughgarden, Bruce Bagemihl, Thierry Lodé and Paul Vasey suggest the social function of sex (both homosexual and heterosexual) is not necessarily connected to dominance, but serves to strengthen alliances and social ties within a flock. Others have argued that social organization theory is inadequate because it cannot account for some homosexual behaviors, for example, penguin species where same-sex individuals mate for life and refuse to pair with females when given the chance. Homosexuality is a social phenomenon and is most widespread among animals with a complex herd life.

A first-ever museum display, "Against Nature?", which opened October 2006 at the University of Oslo's Natural History Museum in Norway, presents 51 species of animals exhibiting homosexuality. The argument that a homosexual way of living cannot be accepted because it is against the "laws of nature" can now be rejected scientifically, said Geir Soli, project leader for the exhibition. "A main target for this project was to get museums involved in current debate; to show that museums are more than just a gallery for the past." Geir Soeli also said: "Homosexuality has been observed for more than 1500 animal species, and is well documented for 500 of them."

Also masturbation is common in the animal kingdom.
"Masturbation is the simplest method of self pleasure. We have a Darwinist mentality that all animals only have sex to procreate. But there are plenty of animals who will masturbate when they have nothing better to do. Masturbation has been observed among primates, deer, killer whales and penguins, and we're talking about both males and females. They rub themselves against stones and roots. Orangutans are especially inventive. They make dildos of wood and bark," says Petter Boeckman of the Norwegian Natural History Museum.