Woe to us poor bisexuals who come to poly thinking that it is the end solution to all our life/relationship problems, only to find it does nothing but complicate them!
One of the most common triggers for jealousy from what I've read is the fear of being replaced. Another very common one is the related fear of not being enough, good/smart/hot/passionate/feminine/masculine/whatever enough for our partner. If you look at it rationally, you are not a generic female who fills the place for a female archetype, 'The Girlfriend', in your bf's life. He is with you because you are the person you are, not because you are female. The same goes for your bisexuality. You don't want to be with ANY man, you want to be with him.
I'm not saying you need to adopt the 'Love the person, not the gender' approach to bi/pansexuality. I care very much for the gender identification of my partners. However, to me, the thought that there would be one place for a Generic Manly Man and another place for the Generic Girly Girl in my romantic life is pretty absurd. That is also why I don't get one-penis-policies. Connections happen with people, and just because you have one of each gender doesn't mean you can't connect with anyone else or only with someone of the other gender tag because somehow magically the 'slots' in your emotional life have been filled.
Your boyfriend sounds like someone who isn't interested in poly but in some other form of non-monogamy, but you can't really escape from having to gain some basic jealousy management skills either way. I warmly recommend getting at the root of this jealousy problem before anyone adds any more partners of either gender.
Me: bi female in my twenties