As life in general constituted much pain in the form of struggles against poverty, disease, ignorance, and emotional anguish, what more civilized way for people to alleviate the same than by giving themselves to one another as brothers and sisters in deed as well as in word? A society of people hoping to become politically superior needed first to become spiritually valid.
The woman who thinks she can choose femininity, can toy with it like the social drinker toys with wine - well, she's asking for it, asking to be undone, devoured, asking to spend her life perpetrating a new fraud, manufacturing a new fake identity, only this time it's her equality that's fake.
Knowing there is a world that will outlive you, there are people whose well-being depends on how you live your life, affects the way you live your life, whether or not you directly experience those effects. You want to be the kind of person who has the larger view, who takes other people's interests into account, who's dedicated to the principles that you can justify, like justice, knowledge, truth, beauty and morality.