I'm so sorry OP, I really feel your heartache as I'm in a very similar position myself.
My DH has become friends with a female colleague, they spend a lot of time together, I'm insanely jealous (of the time and his attention) and he knows that I'm unhappy about the situation. But he insists it's "just a friendship", there's no reason for me to feel jealous and he carries on seeing her.
Do I try to believe him? Do I just accept that in this instance he prioritises this friendship and his feelings over mine?
I don't want to dictate who he can be friends with, but this friendship - because she is an intelligent, younger female? - has upset me so much. And it continues because it's purely platonic, apparently. And I continue to feel upset every time he sees her.
My DH hasn't given me an ultimatum, nor I him. But as your husband has forced a choice upon you, I wonder if he isn't aware of exactly how damaging his friendship is to your marriage. I like the idea of putting things in writing, as another poster suggested.
I'm afraid of being made to choose or of asking my husband to choose - I'm not prepared to possibly break up our marriage. Call that a lack of self-esteem, fear of being alone, preference for being in a stable-ish, mostly-happy-ish relationship, I dont know.
All strength and best of luck to you OP.