I know this is the Ask Male thread and I think the guys have had an excellent input. As a woman, I just wouldn't put up with this BS. He says she is his "best friend"? Shouldn't you be his best friend? Seriously, there is a lot more going on in their relationship than he wants you to believe. They have keys to each others apartments for an emergency? What kind of emergency? You have asked him to choose and he won't? If you were friends with your ex and your love was really uncomfortable with it, what would you do? Plus, this isn't just a casual, lightheartedness relationship. I had a child with my ex and we are not that friendly!
I do not think you are being unreasonable with the way you are feeling or the things you are asking of him. It doesn't matter how long he has known her. They are no longer together and his first priority should be you. Nothing else makes sense.