Who we are is complex. It incorporates many aspects of where we grew up, how we identify ourselves, how others identify us, actions we have taken in the past and who we would like to grow into. Identity can be fairly solid, it can shift fluidly, or migrate through a series of stages.
Once aspect that is a key component to how we see ourselves and how society sees us is biological sex. I was going to write a big thing about it, but then SciShow on Youtube did it for me.
This episode was written by Carly Britton and covers many aspects that are worth listening to.
In short, Biological sex is not binary, it is a spectrum.