It's really not that complicated. Sex is what you're assigned at birth based on your characteristics. no one can know what is in your brain (or wherever you "feel" who you are). Transgender people "feel" that they are the opposite sex of that assigned to them. There is a mismatch between their sex and their gender identity. For those of the religious ilk, which is not me, I guess I would characterize it as the soul feels it doesn't belong in the body where it resides. Orientation is completely unrelated to gender identity, as I'm sure you understand. I'm not sure what to say about preferences. And in some cases, even transitioning successfully to the opposite gender will not "fix", and I hate that word, the mismatch, because, well, it is simply not enough. The outside may be fixed, and part of the inside (think a male having vaginoplasty), but the "rest" of the insides are not, and cannot be. In these cases the only thing that would, could be acceptable is being a cis-female, an actual female from birth, and we know that this is not possible. Hence the dysphoria. There is no "fix" for having these feelings.