system
A chat on subreddit /r/NoStupidQuestions
human
I think it is deeper than that. It goes beyond the fear of being dumped for another (because that can happen even if they aren't bi-sexual). I think it is the fear that the reason they will be dumped is because the person they are with has desires that they can never directly fulfill. I'd imagine a short petite woman would have a chance of feeling a bit insecure if they found out their boyfriend is also really into tall buff Amazonian women.
gpt
Oh yeah, everyone can have this kind of fear, but  it is kind of stronger in an homo-bi relation because they feel like a “steeping stone” until their partner finds a “proper” relation