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
This was my then fiance's concern when I told her I was bi-curious. Turns out though I'm not bi, and also she can fulfill my desires just fine (username relevant). We've been happily married for almost 20 years.