I find it far easier to think in terms of the fuller expression: sexual reproduction. Humans reproduce sexually, and not, for example, asexually. This means we require a male and female gamete to create offspring. So sex is which gamete your body was designed to produce. I then round this out with a second element which is to recognize that humans are bipedal, but not everyone can walk. The point being the design laid down in uterine development to produce a specific gamete defines sex not the ability. Lastly this is biology, not sociology, when we talk sex. If we want to talk gender we bring in the latter. I find these core concepts have helped me and I hope they are helpful to others and would welcome other thoughts and ideas.