If a person who suffers from gender/sex dysphoria transitions, is it a sin for them to transition?
To my mind, transgenderism/transsexuality poses a problem to Christian anthropology as regards the sexes. Traditional Christianity teaches that God made humanity as male and female. That is the primal identity for a human being. Everything else follows from that. So your soul is either male or female because the body and the soul are both required to make up a complete human being (hence the notion of the Resurrection over something like reincarnation, which treats the body as a disposable facade). So if you're a man, you're a man because your soul is male and because your body proclaims you ad such. It's an intrinsic reality. Same for females.
So how are transsexuals supposed to figure into this? The best theory about it that science has is the brain of the fetus is hit with a large amount of the hormones of the opposite sex while in the womb. This causes the brain to either masculinize or femininize. It creates the dysphoria since the brain expects the body to be one thing but it's the other. Only hormonal and surgical intervention has been proven to help, so far.
In this view, transsexualism is a sex differentiation disorder, a neurological condition - a medical condition at the root of it. It's not a mental illness or a delusion. (I'm only speaking about transsexualism here because I don't really understand things like non-binary, degender, trans people who don't want to transition or whatever else that's under the "trans umbrella" and science hasn't said anything about them, as far as I know. It's not my reality, either.)
So how would such a person fit into Christian anthropology of the sexes? Were they meant to be the sex that is encoded in their chromosomes and it is destroying God's creation to tamper with it? What about the fact the brain more closely matches the brain of the identified sex? You can't just throw the science of it out, emerging as it may be.
Can a male soul become incarnate in a female body and vice versa? People are born with all manner of congenital birth defects and other mishaps, including being intersex which transsexualism may well be a sort of intersex condition. Christianity teaches that all bodily defects and problems will be fixed after the Resurrection when humanity receives glorified bodies. Might that be the case for transsexuals? Might we finally receive bodies that match our brains and souls?
All I know that is Christianity had better start thinking about this issue in a logical way. I have seen no real attempts from the Catholic or Orthodox Churches to address this issue in a theological way. If they say that it is a sin to transition, then you throw the science out the window and condemn transsexuals who have transitioned to lives of being outcast from church communities and humiliation. After all, after a certain point in transitioning, you can't really detransition. If you say it is fine, you still have to square it with the theology surrounding the creation of male and female.
Personally I see being transsexual as a medical condition or a birth defect of sorts. I'm not really proud of it and just wish my body and mind were in alignment. I don't think of it as spitting in the face of God (I love God even if I don't understand Him/Her), but as treating a medical condition. If the Resurrection is true, I pray that God will have mercy on me and other transsexuals.