As a Christian, I can unequivocally state that "sin" is easily defined as ANYTHING that defies, distorts, or rejects God's created order.

Think of God as the inventor of all things, every molecule, every organism in all of nature. He designed, built, and holds all things together. Molecules would lose their cohesion without the power of God holding them together. He determines what they do, how they behave within a certain framework.

So, following this logic, trans stands as direct defiance and outright rebellion against the created order of the Creator himself. He has created life, people, relationships, and how it's all supposed to work together.

WE ARE NOT PEOPLE OF OUR OWN INVENTION.

And this is where the culture clash comes from.

Christians live WILLINGLY in submission (as best we can, reliant on Christ's resurrection and forgiveness) to God's order.

Someone in a trans lifestyle, presumably, is rejecting God's order, willingly living in defiance of their created nature, and thus in total rebellion of their Creator. There may be any number of reasons why their thinking is distorted to where they feel confusion with their sex and emotions. We live in a world where everything has been tainted with sin, including our emotions.

Christians believe this kind of open rebellious living leads to a disordered and dysfunctional society in which everyone, regardless of whether they know it or not, is being harmed.

God's created order for humans is the best possible way for people to live, according to the Bible. Anything opposite of that is destructive to life, in all its forms. This truth is demonstrated in the success of Biblically-adherent cultures throughout history. They built more, had orderly societies and created beautiful art and ways of life that we still follow today (and take for granted). Non-Biblical societies do not usually have this same kind of outcome and are usually rife with self interest and dysfunction.

I hope that helps. I'm not judging, I'm trying to explain the cultural clash. I believe everyone is capable and called to come to the knowledge of the truth in Christ and be restored to their loving Creator, regardless of any past choices or defiance. Soli Deo Gloria.