Nothing really works as an example, that's sort of the point though. OP's girlfriend is straight (I'm assuming), she doesn't know what it's like to be bisexual and if she hasn't had other bisexuals in her life then she doesn't really have any frame of reference for what it's like.

Yes, people assume that their partners are capable of being attracted to other people, but how often to people make it a point to specifically tell their partner that they're attracted to other people (celebrities don't count because they're unattainable, the attraction to them isn't a threat to the relationship)? Outside of a Coming Out context, not many.

She is ignorant. She doesn't understand why it's relevant to their relationship because she's clearly never experienced it herself so she's feeling insecure and wondering if she's enough and if maybe the reason he was telling her is because she isn't.

There is a lot of ignorance around bisexuality and when people don't personally know any bisexuals, they often believe stereotypes about us or they will make comments and judgements without even realising that they're being biphobic. It's great that you would understand the need for your partner to have that conversation but not everybody is at that point, not everybody is informed on why it's an important conversation to have.

Not everyone I've come out to has reacted perfectly. It would've been easy to just write them off as bigots and cut them out of my life, but had I done that I wouldn't have some of the amazing people I have in my life now. It sucks, but sometimes people just need a little bit of patience and for things to be explained to them first and to be allowed to ask questions. If someone reacts badly but is open to learning, they deserve the chance to be better. If someone persists in being biphobic after I've explained it to them, then they deserve to be cut out of my life.