Human sex differences are binary, as is human reproduction. Sex differences are based in biology and genetics and, at this point in scientific development, are immutable as they are present in every cell of the body. The so-called contemporary sex transitions are actually designed, by our very visual culture, to affect outward appearance within the binary designation and often do just that with the aid of hormones and sometimes surgery, along with variations in hairstyle, facial hair, and makeup/clothing. Several of the blind people who participated in my studyacknowledged that they could not perceive transition in a friend or acquaintance because the change was visually-based.
In truth, sexual transition does not and cannot exist until science is capable of completely transforming every cell in the bodies and relevant aspects of the genome.
Gender transition is entirely possible. Sex transition is not. However, given the gender fluidity being presented at the current historical moment, many think it unnecessary, while others still decide to go the full way to adopt a different identity.
I will not try to explore the personal motives for each choice at this time. They are most likely as different as the individuals themselves are different and would take a complex and lengthy analysis. I do, however, want to note one overriding and important difference that is at the root of much of the dispute about these choices.
The choice to live one’s gender fluidly is the more radical and political choice in that its focus is to abolish the arbitrary role differences assigned to males and females. The option to transition leaves the socially constructed gender roles in place and attempts instead to change the place of the individual in that hierarchy. The former emphasizes societal change, the latter individual change.
Psychotherapy must deal with the personal, as well as the cultural context and should be helpful in sorting out these issues for individuals contemplating either of these choices. The therapist should also be carefully trained to help explore these issues. There are very complex personal and cultural reasons to engage in these profound and, in some cases, irreversible decisions and this is definitely a situation requiring depth, gender analysis, and personal/socio-cultural analysis, along with the maturity to understand well the temporary and permanent ramifications of such decisions.
 Sight Unseen: Gender and Race through Blind Eyes, Columbia University Press, 2015.