The book I am thinking of starts with a boy and girl (I think they are class mates but not friends) both getting the same fantasy game then playing it at their own homes.
When they started the game there was a character selection screen with a a bunch of your generic fantasy class/characters. The twist was that the videogame characters were in on that the kids were going to get transported into the game. The sexualized male and female warrior characters were designed and supposed to lead them through the adventure (We don't learn this for a little bit).
I remember that the boy picked the sexualized adult female warrior character, because he's boy, duh. I think that the girl chose the anthropomorphic character (squirrel person maybe?). The important thing with the (squirrel?) person was that they were (technically) a child. The squirrel child mentions this regularly when anything adult happens and how he can't legally view the adult thing because of "X" Corp, the makers of the game I think.
This throws the other classes for a loop since she didn't pick the character she was supposed to, though it doesn't ruin anything permanently.
There was also something about getting a wish or your hearts desire if you win the game.
I remember the book being trippy, weird, and self aware.
Some of the plot points I remember:
There was a village where a machine was stopping everyone from cursing/saying sexual things. For example, a villager explained that he couldn't complement the size a female warrior's bosom/breasts. When he tried to say the word it would be "bleeped" out. The kids eventually rescue the village from that horrible fate.
The kids help a female mermaid get back to her husband. When the mermaids embrace and wrap their tails around each other the squirrel child character says "I am a child and cant view any sexually explicit material because...".
The girls finds a dog and takes it back to the real world when the game ends.
Throughout the adventure the girl and boy become closer and eventually the girl winning the game (not sure how). When she gets back to the real world she finds she has everything she wanted (A pet dog and a nice boyfriend).
I remember reading the book when I was young and it constantly surprising me. I would love to read it again.