Look at this from his point of view.
You are being taken advantage of, as far as he's concerned, and the fact you seem to enthusiastically accept this arrangement is deeply baffling. Your boyfriend is going to leave you and break your heart--he's already in the process of doing it, because there's somebody else!--basically it's a giant what the fuck.
Many mono people just can't process poly right away. They insist on seeing it as cheating...at best, they think you were coerced into "letting" him cheat. It's the only cultural framework they've got: every love triangle in literature or film MUST resolve into one happy couple and one person left wanting.
It takes time. It took my dad and stepmother seven months. When I announced we were poly, they were LIVID. He started with "so why'd you get married, then, if you were just going to fuck around?" (We'd been married for almost fourteen years at that point, and talked about opening up almost as long.) My dad is a retired cop and when he gets into interrogation mode, it can be intimidating. I let him vent for a while and then I said "Dad...you've always told not to judge people until I've got all the information...when do I get a turn?
I calmly explained this was a mutual decision, many years in the making, and that we were both very happy about it. That nobody had any plans to leave anyone and as far as he was concerned, nothing's changed.
He insisted on checking in a month later out of concern, trying to get the scorecard on our relationship, wondering why it was I didn't have an outside lover of my own, and I told him in a letter at that point that our sex lives were out of bounds unless he expected me to start asking about his with my stepmother, which I had zero interest in doing.
Time passed.
I wrote several blog posts explaining poly, aimed anywhere and everywhere bit definitely at them.
Seven months later we met for a Christmas lunch and my stepmother, who had previously acted as if my wife was no longer welcome in her home, insisted on a picture with her.

I'm not saying he'll come around for sure. But you might be surprised.