Tantrikas, and esotericists in general, will often tell you that there is a polarity going on in the whole universe, on any level from galaxies down to subatomic particles - male and female energies abound, dance, seduce each other. Beautiful imagery.
I won't even go into why it makes no sense at all to assume that the physical universe conforms to human ideas of polarity. Somehow, if the universe is dualistic, and humans are dualistic too, that seems to "connect" one to the other in some fashion. Whatever... Let's just let that one slide for the moment.
Of course, male equals active, penetrating, aggressive - essentially, all the evil but desirable things that we crave for but don't admit to - while female equals passive, receptive, nurturing - all the good things nobody really wants to be.
They will then tell you that, of course, this is not a gender stereotype, since everybody has male and female qualities.
Well, duh - then why do you insist on calling it "male" and "female", "shiva" and "shakti" etc.? Why not simply call it positive and negative, or active and passive?
I think the answer is shockingly simple: It creates more emotion, makes for better imagery, it is easy to relate to, one can create wonderful stories about it and, and, and... wait for it... well, yes, it IS a gender stereotype. Tadaaaaah.