when I "change stances" or penetrate, the girl I'm fighting doesn't actually change, she just stands there, is that supposed to happen?
most characters won't change pictures when changing stances. The stance they are currently in will show under the character sprite.
wait, so then when do they change pictures?
They don't change pictures, for the most part. There are only few CGs of the kind you're talking about. This is a text-based game, as this very page states.
Bro is a text-based renpy game, not a 3d porn game