True, Timber, but it's not just that, I think.
The pure hatred, revulsion and disgust some people feel towards certain groups is much stronger than a simple instinct to divide people into groups and establish to which group they belong themselves.
Technically, I divide people into men and women (that is, I know which ones are male and which are female and subconciously make a distinction. Although I do live in Amsterdam where it's sometimes hard to tell
). But I don't hate men or regard them as "the Other". What you describe is probably the first step to a much more complex problem.