The first observation: Jealousy is a common theme across relationships. There’s probably a TV Tropes page for it, but I’m not going down that rabbit hole tonight. But I imagine everyone is familiar with the stereotype of one (usually straight) partner forbidding the other from interacting with anyone of the same gender/sex as the forbidding partner. E.g. a girlfriend telling her boyfriend not to talk with any other women. I once saw someone suggest no men should ever spent significant time with any married women.
That sort of thing doesn’t really work for bisexuals. To forbid spending time with anyone else is just absurd. So the only option is to acknowledge that your partner may have friendships with people they may, if they’re not in a relationship already, date.
The second observation: At least according to what I’ve seen, monosexuals have an entire gender of people that are simply beyond the realm of romantic or sexual attraction. Thoughts of those varieties simply don’t come up seriously. (This probably has a lot to do with the possibility of the first observation.) Bisexuals of course still have some people whom are not seriously considered those ways (because of age, familial relations, etc.), but the pool is much smaller. The assumed lines are much fewer, so more consideration is required.