@freemo Another interesting conversation! (Especially for a dabbler in symbology). I think the fundamental difference is that femininity (associated with water or geometrically, the circle) is a spiritual stance of accepting information; which is why it is associated with empathy because to do so you must take in the information being given by other people in order to empathize with them. Masculinity (fire or geometrically a line) is the opposite, a blocking out of information so that one can act (since the world produces a huge abundance of information that we cannot possibly process, we are required to block out some to not be overwhelmed all the time). Thus action (getting from point A to point B, like a line) is considered masculine and reception (ability to contain like a circle or a receptacle) is considered feminine. What is unclear to me is whether the association to human sexes came from the geometric association (the main distinguishing feature of the sexes, the genitals, resembling a line for men and a circle for women) or if some observations of generalities in human behavior led to the association (men being more prone to action without as much thought and women being more likely to "overthink" things, or at least think more than men do). That is my concise answer, believe it or not
@freemo I agree with a lot of that. You also reminded me of another reason I believe it became associated with the division of human sexes is that the two energies are required to be in balance with each other for the act of creation (perhaps seen as positive organization of matter) to take place, just as both male and female bodies are required for procreation. This is opposed to the imbalance of these energies which causes dissolution; excess male energy is how we view aggression symbolically and excess female energy as anxiety.
@Clementulus Yup, that was very intentional in my interpretation :)