My boyfriend recently started seeing this person who doesn’t agree with polyamorous relationships and my boyfriend has liked them for a very long time. He told me if they wanted a serious relationship he would be monogamous for them. This thought has made me very uneasy. I don’t think it’s fair to neither me nor his other partners? I’m fairly positive his other partners don’t even know about this. What should I do? I love him but I’m really scared he’s going to leave us all for this crush.
You can’t control your boyfriend’s behavior: if he decides he’s going to leave his polyamorous partners to pursue a monogamous relationship with this new person, he can do that.
Your choice is whether you feel comfortable staying in a relationship with this level of risk attached. If your boyfriend has flat out told you he’d leave you for someone else if that person asked him to, you’re well within your rights to consider that a dealbreaker rather than live in fear, or worse, competition. You don’t have to, though - if this is a level of risk you’re willing to tolerate for now, that’s your call.
Just like someone in a monogamous relationship is free to leave that relationship to pursue polyamory, someone in a polyamorous relationship is free to leave to pursue monogamy. This is usually devastating to the partners who get left - breakups always hurt, for whatever reason. Sometimes it works out for the person, sometimes their calculus was off. But everyone is free to pursue whatever path they think is best for them.
I do think that if you want to, you’d also be within your rights to push your partner to tell his other partners that this is percolating, so they can make the same informed decision you are able to make.