This is from the chapter about monosexism and biphobia, from the sub-section about internalized biphobia (and the sub-sub section about internalized biphobia in intimate relationships). I wrote about three types of internalized biphobia inside intimate relationships; this is the second.
Similar to social settings, internalized biphobia might also influence people inside intimate relationships in a way that is disruptive and harmful both to the relationship and the people within. Inside relationships, some bisexual people might treat their partners in ways similar to those of biphobic monosexual people, as informed by stereotypes about bisexuals’ dishonesty and lack of loyalty, as well as returning to some of the basic underlying themes of internalized biphobia such as lack of acceptance and worthlessness.
[…]
Bisexuals who […] choose to date other bisexuals might […] be influenced by internalized biphobia, often in subconscious ways. Many bisexuals people might fear that their partner might cheat on them or leave them for a member of another gender. If the relationship is nonmonogamous, then people might need to deal with more jealousy or a feeling of being threatened when their partner hooks up with someone of a particular gender. For example, in one of my relationships, my partner and I needed to deal with her internalized biphobia when I started dating a man. She feared that he might be able to satisfy me in ways that she couldn’t and that I might leave her for him (despite our polyamorous relationship). True to the under-the-radar character of internalized biphobia, she didn’t realize that this is what it was until I pointed it out.
I’ve found that these sorts of fears are more often triggered by (potential) male partners rather than anyone else, and regardless of the gender of the person in the relationship. To simplify: whether the person inside the relationship is a man, a woman, a non-binary gender or any other gender, these feelings might be more likely triggered by interest in a man than a person of any other gender. This means that these fears are more related to masculinist* and patriarchal** values than to categorical gendered thought (i.e. “this other person is of a different gender category than me, therefore they are more threatening”). The social presumption that males have more value here plays out through the assumption that bisexual people of any gender would always prefer men. Thus, the fear that one’s bisexual partner might leave them for the proverbial “someone with a penis”*** are informed both by sexism (assuming men’s superior value) and by biphobic notions according to which bisexuals are “actually” monosexual. In addition, a contributing factor might be people’s feelings of worthlessness as bisexuals, thinking themselves as undeserving of love and intimacy, or as “not good enough” to be with and thus dismissible by their partners.
* Patriarchy means the system of male superiority and rule.
** Masculinism is the system attributing more value and power to masculine people or to anything else which might be perceived as masculine (personality characteristics, hobbies, interests, social values, etc.)
*** Note that not all men actually have penises (notably, many transgender men). However, popular sexist, cissexist and heteropatriarchal thought often constructs men as metaphorically phallic (or as possessing phallic power) whether or not they “possess” the organ itself.