Those that are are called "dichotomies"
Those situations or categorizations that are not dichotomies, but are cast as such, are false dichotomies.
The use of false dichotomies is a sign of sloppy thinking or rhetorical excess (or, perhaps, something else--saying it could only be one or the other would be a false dichotomy :-)
The inability to detect a false dichotomy arises, perhaps, out of a lack of imagination or an oversensitive sense of the absurd in the person generating the dichotomy. To the extent that each of us has an imaginative capacity that could use some exercise, false dichotomy is something each must work to avoid.
See also Wiki:ExcludedMiddle, Wiki:FallaciousArguments, Wiki:FalseDichotomy, Wiki:BifurcationFallacy