There is a strong tradition in the Abrahamic faiths of being very careful to avoid things that might be sin; this is how you get from "thou shalt not seethe a kid in its mother's milk" to "can't cook chicken in a pan that has been used to cook something with dairy in it".
The US, in particular, has been heavily influenced by a sexual purity ethic view of Christianity, which tends to value very highly adherence to a particular set of visible structures.
My thought: Do your best, but remember that the entire point of the faith is that there is forgiveness for sins. You are going to fall short in many ways, on many topics, and all you can do is do your best and trust God to sort it out. If you look through our past, you will see Christians going to war, and Christians saying that all war is sin. You will see Christians keeping slaves, and Christians freeing slaves. We can't all be right. None of us are right all the time.
Two passages you should read. The first is Romans 14. Think very hard about this one; Paul's assertion is that nothing is sin in and of itself, only sin to one person or another. If meat sacrificed to idols is sin for you, don't eat it; eating it is sin. If it's not, go ahead and eat it. Interestingly, Paul's view is that the stronger your faith is, the more things you can do -- they are no longer sin for you.
The second is in Romans 8. Just read Romans 8:38-39. Whenever you're unsure, go back to that.
"For I am persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord."
There is nothing to fear. You may screw up. You will screw up. But it's okay. Be what you were made to be, and trust God. As Boenhoffer puts it, don't trust God to justify the sin, trust God to justify the sinner.
I don't know whether it is sin for you to transition, or to be with a particular sex of partner. For me, I would not think it would be (well, except that I'm already married, but picky picky). It might be for you. It might not be for you. It is possible that it would be for one of your friends, but not for you. In which case, they may mean well and even be entirely truthful when they tell you they think it is sin, but that doesn't mean you are prohibited from doing it.
Another way to think about it: Imagine that you were to torture Jesus to death, and while He was dying, start gambling over who gets to sell His clothes. How do you think Jesus would react to that? Conveniently, we have the answer written down: He would forgive you. Do you seriously think that anything you are proposing to do is going to get a harsher punishment?
You are what you are; be yourself, and be yourself boldly, and trust God to understand how the way you were made has shaped your choices.
As a side note: When people tell me something is sin, I usually ask them a couple of questions. The first is whether they can make dry bones dance again, because if they can't, they are not actually the boss of me. The second is how many hungry people they've fed recently. I am inclined to feel that anyone whose practice of Christianity does not involve actually directly going out there and feeding people who need food on a fairly regular basis is probably not a credible authority.
