To get this achievement you have to play the game normally until you get to Night 2. During this you will be shruken down and will be fighting the Underpants Gnomes. To keep track, the quest is called "Defeat The Underpants Gnomes". This is a main storyline quest. Anyway, eventually in the quest you will exit from the wall and will be in your parents' bedroom on their dresser. In the background you will see your parents having sex. Simply have your character face them and wait for 60 seconds. After that, the achievement should unlock.