Your feelings aren't wrong, it's how you handle them that matters. It sounds like you're doing a very mature job of dealing with a very new situation. Don't worry about what your friends say, they'll either come around in time when they see he's not going to hurt you, or else they're kind of crappy friends. Even experienced poly people experience jealousy from time to time, but it gets much, much easier, and there are ways to work on it -- check out the essays on the topic at www.morethantwo.com
, they're great.
It may also help to develop more of a friendship with his other gf... it can be really wonderful to get to the point where instead of feeling jealous you feel like partners in crime... I know that probably seems like a stretch now, but it can happen, believe me! Plus it'll make it easier on him if he can hang out with both of you at once from time to time rather than having to totally compartmentalize his life.
Do you think you might ever consider a second bf, if you met the right guy and he and your current bf got along?
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.