cook417 put it well "Its the juxtaposition of those things that drives me wild."
I would also add the feeling that our "male territory is being invaded." As culture, we are taught that men are supposed to be strong, not women.
And yet here we see these hunk female bodybuilders in the gym, grunting and groaning during sets, swaggering around with the their lats puffed out,
giving each other congratulatary fist bumps, while skinny wimpy males cower and look on in awed wonder. Its a role reversal, yet with the women
maintaining their femininity all the while.