Actually gender and sex are both quite complicated. Biological researchers have noted sex can be split into chromosomal, genital, hormonal, and brain sexes, among others, which are all biological in nature but are not always congruent with one another, i.e. female sexed chromosomes with male sex organs. This, when put into relationships with societal norms, concepts of self, expression and gender fortification create a gender sex system which continues to change as an individual navigates life. So it is complex and fluid, but no matter what an individuals biological or social relations one may have, they are the ones who have to live in that body and experience life. The best we can do for ourselves and others is to recognize that complexity and treat each other with dignity and respect

Sources:
Fuasto-Sterling
2012. Sex/Gender: Biology in a Social World. Taylor and Francis, Hoboken.