That sofia fight got me thinking about that too with the final fight if you lose while penetrated. It'd actually make sense to have some monster girls actually be dangerous that could take xp if you lost a fight. Maybe a route to make petua stronger? Convert your xp to a potion that she can take?
Later down the line, there could actually be true succubus in an area that if you lose you get a game over with your soul getting sucked out or does your levels need to be gone before that can happen?