Azusa was getting too boring and she wasn't growing in any way... She needed this, hopefully this new girl will help her stop being a classic "must protect morals" type of girl and be more open minded... Honestly even if that doesn't happen i find this new girl more fun than Azusa so i'm fine either way
Not to mention now we get to see two sides of the coin... One side we have a prim and proper girl being exposed to lewd stuff, on the other side we have a girl who grew up not minding sexual stuff being introduced to warm family and friendship bonds... This IS still related to the setting...