My boyfriend is VERY jealous of my best friend

So my bf and I have a 6mo old daughter. Basically, we were broken up when she was born and got back together shortly after. My boyfriend is such a joy and I light up when I see him. He is truly wonderful and I love him so much. So with that, I also really care about my best friend. She’s been in my life since we were kids and I sincerely love her - platonically.

So I told him I am bisexual several weeks ago which he had an odd response to. He asked me if I ever did anything with my best friend and I told him I had feelings for her in the past and he freaked out. Didn’t really fight but he was weirded out by it.

So my wallpaper on my phone is me, my baby, and my best friend on the day she was born and then on the Home Screen it’s just me and my bf at homecoming. He is so incredibly offended by this and I don’t understand why. Is it really a bad thing to have my wallpaper be the most beautiful day of my life?