Hullo and warm welcome!
One thing that immediately came to my mind is that when you say you are both bisexual, is that something you have worked through? Although as a bi girl who has dated straight men I would LOVE to have someone who gets me at that level and doesn't hold my orientation against me or transform it to a world of insecurity, it seems you have established a pattern where you are not really comfortable with bisexual non-monogamy. I mean, he chooses not only to sleep with you best friend but with your best friend who is also male? And as FlameKat already pointed out, he might feel more comfortable with a one-penis-policy (do a tag search on that if the term is not familiar).
Me: bi female in my twenties