I know the conversation has long since moved on, but I just wanted to point out that the expert, and some commenters, seemed to equate being scared BBB was going to leave her for a guy with thinking he's going to turn/turn out gay.

BBB's gf has a frame of reference bc she can imagine herself in an outside os relationship and feel confident, in the same way other guys ultimately can't compete with him, other girls can't compete with her. The gf may also feel more confident that she can "directly" compete on similar characteristics with other women or that any small differences in characteristics to characteristic comparisons will be made up for by their time together.

With an ss relationship, however, there are certain things the main os partner can never directly compete with. The ss person knows, from a personal perspective, how to pleasure your partner in a way that you or another os person never will. There's also socialization, experiences and personal issues that ss people are more likely to share that an os partner isn't.

Open relationships are always going to involve insecurity and just because those insecurities are different for os vs ss relationships, and maybe one is scarier for a person than the other, doesn't necessarily mean it's biphobia.