BF “friends” with an ex GF

A couple of months ago I caught my BF trying to hide that an-ex GF was over his house. They supposedly dated for a couple of months in the past and she broke it off to sleep with his friend.

I was upset and told him that he needed to stop seeing her if we were going to remain a couple. He agreed. Well, today, I had a gut instinct that he was with her, and he told me he was having dinner with her and it was platonic. I gave him an ultimatum- her or me. I am so confused since he has been seemingly so committed and available to me. I love him - but I don’t know if I can believe they are just good friends. Why hide things if there is nothing to hide? He has been nothing but so loving to me and this is such a huge disappointment. After our fight - he went silent - no texts apologizing or anything. I don’t think I am wrong about this, right?