Question: "How does the fact that there are gay/homosexual animals impact the idea that homosexuality is a sin?"
Answer: Simply put, the fact that there are animals that exhibit what we might call homosexual behavior does not change the fact that homosexuality is a sin for human beings. Some animals eat their young, but that doesn’t mean it is okay for humans to be cannibals. Dogs eat their own vomit (Proverbs 26:11), but that is definitely not good behavior for us to emulate. Some insects devour their partners after mating. We cannot use animal behavior as a basis for morality or to justify our chosen actions.
It is true that “homosexual” behavior has been observed in many species in the animal kingdom. The reasons for such behavior are still being researched, but theories include the need for social dominance, displays of aggression, and avoidance of conflict. Other instances of “homosexuality” in animals can be attributed to their being misled by scent or suffering some physical abnormality. In any case, it is rare to find an animal that displays long-term homosexual behavior. Homosexual behavior in animals is considered anomalous in any species. (See Shlemon , Alan, "Does Homosexual Behavior in Animals Mean It's Natural for Humans?" pub. 3/8/2012, www.str.org/w/does-homosexual-behavior-in-animals-mean-it-s-natural-for-humans-, accessed 7/28/2021.)
“Homosexuality” among animals in nature is not a lifestyle choice. Animals do not operate within any moral context and do not “sin.” However, human beings are not the same as animals. Men and women were made in the image of God (Genesis 1:27). All other creatures of creation were not. Animals are not accountable to God for their actions. Men and women are. Because animals were not made in the image of God, we cannot look at them as examples of how to live our lives.
The Bible is clear that homosexuality is a sin (1 Corinthians 6:9–10). Animals are not going to be condemned by God because of their actions. But sinful humans will be (Romans 3:23; John 1:8). Animals do not have a conscience or an innate knowledge of right and wrong. Humans do. Animals do not have the ability to make moral choices, for they function primarily on a survival instinct.
The fact that some people appeal to the animal kingdom in order to justify their iniquity is telling. Such appeals deride humanity and dishonor God, who has made us in His image and likeness. Animal behavior does not change the Word we have been given or the nature of God who defines good and evil.