Is it a sin to get a sex change? Yes.
However, in rare cases such as Hermaphroditism/Intersex (a person born with both male and female reproductive organs), should be able to undergo surgery on the basis of which of their reproductive organs are more prominent (which can be tested through ultrasounds, blood tests, chromosome analysis, and exploratory surgery). However, that in itself is not a sex change, since a hermaphrodite is not changing from their biological gender to the opposite gender, it can't be considered a true 'sex change'.
The bigger question here is why is getting a sex change a sin?
In my opinion, its because its not addressing the true problem which is first and foremost a spiritual problem; an inability to accept yourself for who you are physically - 'the grass is always greener on the other side of the fence'.
This might sound harsh, but I'm going to be honest when I say that I have struggled with transgenderism myself, everything that I am going to be talking about here is what I have learnt over the years through my own experience (everyone will have their own different opinion on this).
The truth is, that there is nothing wrong with being the biological gender you are. God created both man and woman equal, and in his image. Gender is not to be the centrepiece of our identity, Christ is.
Genesis 1:27: So God created man in his own image,
in the image of God he created him;
male and female he created them.
Galatians 3:28: There is neither Jew nor Greek, there is neither slave nor free, there is no male and female, for you are all one in Christ Jesus.
Which is the other sin here in the picture, the idolization of the opposite gender and what is associated with it in our society.
Romans 1:24-25: Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves, because they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator, who is blessed forever! Amen.
So if God deems both man and woman equal and both made in his image, then what's the problem here? The problem is with humanity and the way we attempt to address issues regarding sexuality. Now I agree with modern science that a person can be born with genetic prepositions that can affect an individuals personality (whether someone is more feminine or masculine -
Sexual orientation and the size of the anterior commissure in the human brain.) but there's nothing wrong with that, Its a part of God's diversity, however if diversity is not treated with respect, it can lead to many problems down the road (which Is what I'm going to talk about next).
However, I think the reason why someone might want to desire to be the opposite gender (or be attracted to the same sex) is due to environmental factors. And this is where it gets personal for myself.
Now, I do fit into the genetic preposition that I was talking about earlier, and prior to certain events I was a very happy little girl. The sort that enjoyed playing with toy trucks, action figurines, climbing trees, playing adventure games etc. And my parents were fine with that, to them I was the classic little tomboy.
However, where it all took an ugly turn for me was when I started school. At school I was sexually abused, and exposed to societies expectations of what men and women (those were the triggers for me). Over the years, first subconsciously, I thought If I were a male I would have more advantages such as being taller and stronger, and people would respect me more. As I grew, these thoughts became more intense, the church I attended during my teens with my family were advocates of complementarianism (not that I see anything wrong with complementarism, although its not the view I hold to), and were not for women speaking in church yet alone holding any position of leadership within the church.
Considering the nature of who I was (and still am) as an individual, and my desire to serve God, I was crushed. And I didn't receive much in support from my church either from when I first talked to the pastor at the time about my transgenderism. I ended up leaving the church in my late teens and haven't attended another church since (the final reason for me leaving was because I was being harassed by some of the female members of the church about my lack of interest in things such as dating, they thought I was a homosexual and accused me of such behind my back).
I thought there was something wrong with me, that God hated me for who I was and what I was struggling with. There was something wrong with me, but it wasn't all on me, nor all my fault.
I felt that I had only two choices;
Fit the female stereotype: play the gender role game that society and the church wanted me to play, ignore the burden I was struggling with and pretend that everything was all good.
Or become a male: start afresh, bury my burden underneath me, ditch the church and possibly my family, and make it up with God later.
The above, were all mankind and temporary solutions. If I had chosen either option, they both would've resulted in death (either choice was form of dress up, a mask).
But little did I know that God had a third option for me;
Let Christ lift and carry the burdens off my shoulders, let him take my sins and wash me anew, let him be my new source of identification. When I turned to Christ, everything else that I had been so caught up in, began to fall away. Before I was in a dark place, in my own world, but Christ opened the heavens for me and the light came through. Through him, I have life and my true identity, and through Christ I can accept who I am.
I can be myself, I can honour the fact that I am a person within Christ, a unique person and a woman. I can humble myself to those (both men and women) who have humbled themselves to Christ. And although I have days where I struggle (we all do), Christ is always there to remind me that I am not of this world and no longer have to conform to the worlds standards, and God's way has the bigger picture in mind, life.