So what's the problem with the quiz? (Besides the sad confusion of sex and gender in the title.) If you guessed the hidden binary, you're right! After all, the possibilities are essentially female, male, or a combination of the two. So whoever made up the quiz was visualizing gender as a line with masculinity and femininity as opposites. They do get extra points for recognizing that between the male and female extremes there might be a continuum, but it still perpetuates a binary, oppositional model of masculine and feminine. Fo years ago, psychologist Sandra Bem introduced a new model that, while still based on the gender binary, made much more sense to me.
My results from the quiz would place me in the middle of the binary model, slightly toward the masculine end. My BSRI results:
"You scored 72.5 out of 100 masculine points, 65.833 out of 100 feminine points, and 58.333 out of 100 androgynous (neutral) points."