An ex girlfriend of mine once told me that when she was a kid at her friend's birthday party she went into the girl's parents bedroom to put her coat away. She heard noises from a walk in closet to find the hostess' (married) father and another girls (married) father having sex. She said she couldn't look at either of them ever again, but never breathed a word of it.

My own experience is while staying in a caravan with my parents as a child and enduring several minutes of holding pillows to my ears trying desperately to sleep.