I just watched this video, and while I have some issues with it, the creator does a fantastic job of addressing why the monster bisexual is such an overwhelmingly common media trope and why biphobia is so central to heterosupremacy as opposed to being peripheral: because bisexuality is "abject" meaning it cannot be confined to either side of an established binary (see Kenji Yoshino's Epistemic Contract of Bisexual Erasure) which makes it instantly and perpetually frightening to society and all its norms. It is specifically because bisexuality cannot be confined that people find it so bizarre, strange and dangerous and thus, conceptualize bisexual people (and intersex or gender non-conforming people tangentially) as inherently threatening and inhumnan.
I really recommend giving this a watch.













