I believe the answer is either-or Fallacy
either-or fallacy refers to a situation where something is falsely considered as one of the two, even though there is a third option that may explain the situation..
Other example, when most people see unattractive guy waling with a beautiful woman, they would automatically think "that guy must be either rich or just a friend" , whle it could be that both of are in a relationship simply because they like each other.