The title says it all. I understand that in this case the acusative case should be used, but why "auf" instead of "in"? Can I say "Ich gehe in die Toilette"?
Because Toilette means in everyday language toilet and not bathroom, so in die Toilette gehen means to step into the toilet bowl. Auf die Toilette gehen comes from auf die Toilette setzen (to sit on the soilet seat)
The room, in which the toilet is, is called Toilette in German (Wikipedia), too. However, I (and the internet agrees) would use Toilette as name for the room only in the sense of "Wo ist die Toilette? (Where is the bathroom)" or "Ich gehe zur Toilette". But in both cases it could also be understand as the toilet itself, too.
If the room contains a toilet and a shower, I would use Bad or Badezimmer instead. For example:
A: Wo bist du?
B: Ich bin im Badezimmer (even though I am actually sitting on the toilet) or Ich bin auf der Toilette or Ich bin auf dem Klo.
"In die Toilette" can be interpreted as into the toilet bowl instead of the bathroom (although people are smart enough to figure out that that is not what you mean).
Here are a few example of "in die Toilette":
- "Nicht mehr benötigte Medikamente gehören nicht in die Toilette oder den Ausguss." (Umweltbundesamt)
- "Mir ist das Handy in die Toilette gefallen (...)" (Reinigungsforum.de)
- "Darf man Speiseabfälle in die Toilette schütten?" (Die Zeit)
Many people use "ins Klo" as a synoym for "in die Toilette", and "aufs Klo (gehen)" as a synonym for "auf die Toilette (gehen)".
The question here is what you want to express.
If you want to say that someone is going to defecate, you are speaking of the toilet bowl ("Toilette") and you do not go into the bowl to defecate but sit on it. The common way to express this is to say: "Ich gehe auf die Toilette." or: "Ich gehe zur Toilette."
If you want to say that someone enters the room ("Toilette") that contains the toilet bowl for some other reason than defecation, you may say both: "Ich gehe auf die Toilette." (and maybe mislead the listener to think that you need to pee) and "Ich gehe in die Toilette." (which will make it clear that you do not intend to use the toilet bowl).
To be more explicit, "in die Toilette gehen" is not false. According to the Duden, you can say "auf die, in die, zur Toilette gehen".
There are many examples for "in der Toilette sein" in the corpus of Die Zeit, for example: "Vor Gericht sagte er, er habe in der Toilette einen Einbrecher vermutet." Here, the room is meant, and "in" is used. Had the author written "auf", the readers would have assumed the burglar was sitting on the toilet bowl.
Other corpora also list countless examples for "in die Toilette gehen", for example: "Nach ein paar Abenden fiel allerdings auf, daß ich in die Toilette hineinging, aber nicht wieder herauskam."
So don't let the other, wrong answers dissuade you from using a perfectly correct German phrase and feel free to say "Ich gehe in die Toilette" whenever you just want to do your hair.