There is little question biological sex is one of the bedrocks of our identity. It's generally the first question we ask of new or expectant parents: "Is it a boy or a girl?"
Gender has always been more flexible though. One doesn't have to look very deeply at history to see that the meaning of male and female has been more fluid than some might feel comfortable admitting.
Dominating the conversation around gender last century was, of course, feminism, the rise of women's rights and the fight for equality: a fight which continues.
A fight to end discrimination based on gender.
A recent Action Station campaign backed by feminist Green and Labour MPs, sought to cover transphobic stickers and posters appearing around New Zealand, with transgender-supportive images.
This discrimination comes not just from the conservative right, or a small group of bigots, but increasingly discrimination by a small group of so-called feminists. "TERFs" is the label given, standing for "Trans Exclusionary Radical Feminists".
At face value it just seems bizarre; that women who self-identify as valuing equality, who fight discrimination based on gender, would so actively - and at times so hatefully - discriminate based on gender.
But an understanding of victimisation and power tells us that those discriminated against can sometimes flip from victim to persecutor. Those bullied can become the bully - and with a surprising ferocity when they do.
From a certain point of view I can understand it. I can see how if you feel you have fought so hard for women that you'd feel deeply protective of your gender. Even willing to fight those who you see as a threat, if you - incorrectly - see them as not really women.
Without fail, transphobia is based on an assumption we increasingly know to be false: that gender is fixed and unchangeable, and if you do transition from being a man to a woman, you are still a man. (and of course, vice-versa).
Psychiatry, always a bit of a slow starter with these things, now recognises those who identify as transgender are not confused or disturbed, but experiencing a mis-match of their self experienced gender, and their biological sex.
Here in New Zealand the Human Rights Commission supports people's right to self identify their gender. That includes on official documents such as birth certificates - a move supported by a recent Select Committee hearing about changes to the Births, Deaths and Marriages Act.
Seen through this lens- and when one truly listens to the experiences of trans people - it's clear that gender is not fixed. Feeling a desire to live life as different to the gender of one's birth is not a whim, or a fetish. It's a person's genuine, deeply felt experience of themselves. People should feel free to express that.
To prohibit that expression, to tell other people who they are, or how they must see themselves, is simply oppression.
And when you find - as happened recently following coverage of this issue in the media - that your transphobic beliefs are being championed by the extreme political right and religious conservatives, it might also be time to consider not only whether you are still radical, but are you still on the right side of history?