most monosexuals think like this because of bi erasure and the idea of a gay-straight dichotomy. it's like if you're from a culture where "purple" was never really a color, and then when it catches on people start thinking of it more as "red-blue" than a color in its own right. also, a little more complicated than that:

think of a bilingual person who speaks both spanish and english. are they somewhere "in the middle" between spanish monolingualism and english monolingualism? no, that would be like, spanglish. bilinguals know both, leading to their own unique experiences/challenges/ways of thinking

edit: also, despite the prefix, bisexuality isn't restricted to 2 genders