Let me just say, if I were the new guy and I found out that I was expected to be monogamous with your bf, even though he 1) has you, 2) isn't prepared for an emotional relationship, 3) might want to be with other guys, I would just laugh my head off and walk away. Hell any ONE of those things would be enough for me to say "wtf, no, that's a ridiculous double standard." I would encourage him to let go of that thought before even bringing it up to the new guy, because it's frankly just offensively condescending.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.