While playing the section where you are shrunken by the Underwear Gnomes, you will see your parents having sex. You will first catch a glimpse of them when they start through the grate, this is NOT the section that you have to stand still for.
When you exit through the wall outlet on to your parent's dresser, this is when you MUST stay still. As soon as you are inside their bedroom looking directly at them, stay still for a full minute. As soon as you see the achievement pop up, you are free to carry on.