Evolutionary hypotheses of sadomasochism with dominant male is quite clear, with dominant female unclear and with homosexual partners equally unclear. And there is no commonly acknowledged psychological explanation of sadomasochism.
So, I don't f*king know! I remember playing a sm-type game with female LEGO figures* before school-age (and puberty ?!) . It's was a some kind dentistry scene, so I blame odontology.
And why I do my silly pictures now? -Because it is fun, that's why.
I read Benfan's comment and edited the box, rendered it, and looked it, and think: "This doesn't make any sense". Then I read the comment another time... Dyslexia strike again! Benfan's version more useful, believe me.