The concept of sexual deviancy cannot be applied to animals. I'm sure nobody would argue such a thing.

To steel man the idea of sin, when the concept came to light, I would have to say: It was a simpler idea to promote better behaviors that resulted in more cooperative cohesive societies. Our social feature to collaborate at scale, made it necessary to come up with ways to keep some sort of order.

Now, if its the best way to do so, that's debatable. I'd say maybe not anymore. The concept of sin is somewhat outdated too. Sin of thought, for example, seems to me impractical.

I don't think the goal of religion is to strengthen the natural order of things. The goal is to make you fight your instincts to show obedience to their god. It's easy to give into your instincts, while it's hard to fight them. So, just because something is natural that doesn't absolve it from being a sin in the churches manmade rules.

It's natural to want to have sex with someone you find attractive, but is considered a sin if you act on those instincts outside of certain manmade rulesets, the same goes for killing as well. I don't think your rationale holds up to the religious doctrine you are addressing. That all being said, I am an Atheist looking from the outside in.

Also, in an attempt to prove our superiority and as a separate creation than animals, the church needs rules that separate us from the animal kingdom. That would be another argument against the stance you've taken.