it's not the rape that's supposed to be funny here, it's the fact that a woman has the ambition and spirit to do something righteous - the rape is just the 'natural' consequence of her thinking something so 'stupid'.
'a woman wanting to be a hero is ridiculous because she's ignorant, naive and weak and of course nothing but endless serial rape will happen to her as a result - if she's lucky she gets paid tho!'
these ideas are not geographically limited to japan...