Why does homosexuality exist?

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I just recently saw two dogs of the same gender(male) taking turns and such imitating sexual intercourse and I guess it just made me think. Why does homosexuality exist? What's the reason? Why? How did it come to be? Why does such a thing exist? Is it one of their archetypes dominating or do you believe in archetypes? Does it exist because it just can? What are your hypothesis on this?