As the Apostle Paul said, we have two natures - one flesh, and one spirit. He described the daily battle he had with the flesh inclination, and said in Romans, 7:19 For the good that I would I do not: but the evil which I would not, that I do.
20 Now if I do that I would not, it is no more I that do it, but sin that dwelleth in me.
Anyone that has watched their dogs see their animal behaviors, and thus see the things that that they do that are similar to themselves. So, is it a sin to do what an animal does? No, that is his nature. So Paul admonished us to seek after that spiritual nature that we all possess. To stay mired in your animal nature seems to be a sin to me, I I personally do not believe that God Created us to stay as animals. He did create us with two natures, so that we would have a choice, where animals do not. So, is it a sin? If you define a sin as rebelling against God’s wishes for us, then I suppose it is. But homosexuality is not elevated beyond gossip, envy, gluttony, etc., or any other act of rebellion against God. They are all evil in God’s eye in my humble opinion. Likewise, to encourage animal behavior is a sin I believe, and ultimately we will be judged on that.