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There is homosexuality in all of the animal world, even in mammalians.

Well that's just not true. We see homoerotic behavior is some animals, not all.

That [anyone] would rely upon animals to 'teach humans' anything betrays its fundamental lack of any moorings or for that matter, of much in the way of common sense. To see this, let us first remember what animals do.

Animals, among other .criminal' activities:

Kill and eat members of their species . that is, they are often cannibals;
Kill and eat some of their offspring . that is, they commit cannibalistic infanticide;
Compete with other males for the opportunity to have sex with a particular female, often killing or debilitating the rival;
Fight with and exile or kill the .owner' of a .harem,' kill his .children,' and then raise other children with the females in the harem (the females show no .loyalty' to their former .husband' and blithely have sex with the new guy).
Discriminate against and kill .aliens' to their particular territory or tribe;
Often have sex with some of their offspring;
Often have sex with relatives and near-relatives (from a human perspective, aunts, uncles, brothers, sisters, grandparents, etc.) . in other words, they commit incest;
Let the males just about always get their way (e.g., eat first, win any spats),
Often allow large groups of males to have sex with a female, even leaving her near-dead or even dead . that is, they engage in .gang rape;' and
Often allow one male to hog all the females in a given area, driving away his .sons' and his .friends' who would like a little sexual .pleasure' with the .girls.'

None of these activities inform how we regard cannibalism, murder, gang rape, marriage, the rightful place of the sexes, or incest. What happens in the animal kingdom is irrelevant to what occurs in the human realm.
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Humans are different from animals in countless ways. Humans speak, write, have a sense of personal history and the history of their culture (and often of other cultures as well). They think about and plan for the future, have theories about what makes the world tick (e.g., believe in God, Evolution, or alien visitors), and devise schemes of how the world 'should be' (e.g., have legal systems and cultural ideals 'everyone is equal', only a man and a woman can be married,'etc.).
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Humans also change the world in innumerable ways (build roads, buildings, bridges, tunnels), and manipulate the earth for food and entertainment (e.g., horticulture, husbandry, planetary exploration). Animals, as near as we can determine, do none of these things. Even when they 'use tools' (like sticks or rocks), animals display only rudimentary skill at the enterprise. Source
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If we arbitrarily pick which animal behaviors to value, then any such study from which we get insights for human behavior is a complete waste of time.