I think of sex as mostly binary and gender as pretty much a continuum. There are intersex individuals and more of them than cursory midwife inspection may suggest; some intersex features only reveal themselves later, often in adolescence. But it's still a small number and to treat sex as binary has always been a fairly good approximation for practical purposes; it has high predictive value for such things as who will get breast cancer and who will get prostate cancer. Worldwide, it also predicts which foetuses are more likely to be aborted: if the scan shows the foetus female, then that is more likely to be aborted.
But gender characteristics are distributed along a continuum rather than distributed into a binary of trans and cis. Most boys, by whatever are the accidents of their upbringing, acquire some feminine characteristics and most girls some masculine ones. Some children end up more thoroughly transgendered than this but manage to find niches in life which suit them.
But I doubt that those who change sex in this way end up completely cisgendered in relation to their new body. They will still have gender characteristics left over from the sex they are leaving. It can get very complicated and that’s fine, though complicated does not necessarily mean heroic or morally unproblematic. You can get rid of parts of a male body to make yourself more of a woman but it's still possible that you act like a bully.
© Trevor Pateman 2019. First published here April 2019