But that wasn't the question. The question was "explain what is sinful about being gender dysphoric" want a discussion? Answer the question you were asked, not some imaginary question you wish you were asked.
I have been doing a lot of research into this question and though it was not directed at me, let me offer my understanding here.
Scientifically, when a baby in in the womb developing, a chemical change occurs in the brain when the male genitals are developing. This means that it is possible for something to go wrong when the chemicals being released are developing gender in the fetus.
Now, take that to the biblical understanding of man, an understanding that is only in opposition to the world understanding when it is to our advantage to change it to justify our sins. According to scripture, man has 3 parts, the body, the soul, and the spirit. The soul and spirit are very closely connected, so close in fact that it is very hard to separate. The body, is the flesh, it is dust, perishable, however you want to view it. Like above, it is an outward appearance only, a home for the soul/spirit. The soul/spirit is eternal, lasting and is where the essence of a man resides. It is the soul/spirit that makes a person who they are.
When it comes to the transgender person, it translates into this. The soul/spirit is neither male or female. Thus when we lack contentment with who we are, it is sin. When we allow our flesh to dictate who we are, or who we are not, it is sin. When we allow the pleasures of the flesh to draw us into sensual pleasures that are not in line with righteousness, it is sin (think here about some of the sexual things as well as a few others but sexual is the easiest to see) When we doubt the wisdom and grace of God to create us as He sees fit, for HIs purpose, it is sin. You see, there are lots and lots of ways that the transgender person can and usually does sin. But that is no different from everyone else in the world. It is possible to have gender dysphoria and not sin, just like it is possible to be homosexual and not sin, but it is very difficult and very rare because we are a people who excel at justifying our sins.
Now, we aren't done, as most people will tell you, it is not a simple matter. We still have the issue of the brain and the body being out of sink. This is a very different matter and given that it is a chemical issue, easy to understand how and why it could happen. The first thing to understand is that this would be unbelievably rare, in fact, so rare as to not require a "movement" to gain rights. Secondly, the correction is not necessarily (notice I did not say absolutely but not likely to be) transition of flesh, but rather renewal of our minds in Christ, just like scripture tells us to do. You see, we can and are told in scripture to control our thoughts and desires. This self control is so seldom taught anymore that it has become scary. So before we even begin to talk about transitioning the flesh, we need to 1. learn contentment and understanding that we are not the gender our flesh dictates and 2. we need to learn to have self control with renewal of thoughts and desires in the power of the Living God.
If after all that, there is still a problem, it would be such an anomaly that we would have to look at chemical imbalances (of all kinds not just male and female hormones) study the brain changes and work within that etc. before considering changing the flesh in ways we cannot really change the flesh only pretend that we can.
So, yes, the transgender can be without sin, but only in the power of the HS, only in denial of the sensual pleasures others take for granted. Just like everyone else in the world.