Yes, it is relevant to look at the animal kingdom to determine if human sex-behavior is normal, because humans are mammals, and we act like mammals in a lot of ways. Even though animal behavior is probably not conclusive of human behavior, it is still a good place to start in order to understand humans.
I believe that other animals have not learned from other animals of their species that homosexuality is possible. I personally believe that if those animals also perform same-sex acts, yet know no better, it should definitely be obvious that we as humans also know no better. If other animals do it, there may be a possibility that we did the same things in our more primitive stages.
People just say "I hate X. X is unnatural." Or "That's gross and unnatural" or what ever variation. People are appealing to an empty concept. They are appealing to emotion and using the word "natural" to make it sound like they actually have an argument. "Natural" doesn't actually mean anything, unless you are going with the more technical definition in which case if you insist on everything being natural please get off the internet, leave your house, and go find a cave in the wilderness and be naked. Anything else would be completely unnatural.
Even then one could argue that "human creativity" in inventing houses and electronics is part of "nature" but then "natural" is defined in a way where everything is "natural" and becomes meaningless.