An antonym for "continuum" to describe a limited set of possible options

Solution 1:

In this particular case you could use dichotomy or division, since you are distinguishing between only two possibilities. (It's not actually that simple in real life, but this is English.SE not Biology.SE).

The best general antonym I can think of is (discrete) set, but you would almost never use it in the same way. You would usually use some more specific term for whatever you were talking about, which would be in turn definable as a set.

Solution 2:

The antonym of 'continuous' is 'discrete'. There is no immediate single word antonym of the noun 'continuum' meaning a set of discrete items (and that would be recognized as such). But since a noun is desired, one can use the word:

alternatives

to be a word that emphasizes the difference with continuum, where you want to say there is a jump between the two endpoints.

Solution 3:

For the original source where the quote was taken from, I think the terminology continuum is fine the way it is. It should have read masculine/feminine continuum rather than male/female, because male and female are usually viewed as alternatives, as opposed to masculine and feminine, where you have a continuum from (to pick two fairly arbitrary exemplars) John Wayne to Marilyn Monroe.