I actually don't like the phrase "real life" when describing what happens in the physical world because it implies that the interactions we have in Azeroth aren't real. Captain Obvious says that WoW is not a single-player game. When we are playing Mass Effect, we don't affect anyone else if we get up to take a bio break or comfort a child. But in World of Warcraft, if we are in a group of any size, we affect others every time we AFK.
The phrase "real life is more important than WoW" is a mantra we hear all the time, and it is true in that you shouldn't shirk responsibilities in the physical world in order to play. However, if you have committed yourself for a period of time to other players, it is the same thing as committing yourself to any group of people in the physical world. Breaking that commitment falls under the same etiquette umbrella, whether in game or out.