The psychological construction of the gender divide is clear and boundaried:
Female = feminine = nurturing, dependent, passive.
Male = masculine = assertive, independent, active.
Our fear of crossing this divide is great. Our sons are growing up paranoid of displaying tender qualities, terrified of appearing dependent. Our daughters are taught to stay away from assertive behavior and sacrifice daring for "good."
While these stereotypes may come naturally to many, they stifle those who simply do not conform.