Your man or bestfriend

Dreka

(SORRY SO LONG) I've been in this relationship going on 4 years. I've been friends with my Bestfriend for almost 10 years... Like 3 years ago me and my Bestfriend was cruising the city and just so happen we bumps into an ex of mine. Now to me its a coincidence BC I haven't talked to him.. He think my Bestfriend set it up. I couldn't answer BC it wasnt to my acknowledgement that we was going to run into him. But anyway it kind of happened twice again not to my acknowledgement but he thinks so. FFWD to a few months ago the same ex gets out yes I did talk to him and also told my fiancé about the situation. Of course he go thru my phone and find me and bestie texting so now he think she the middle man. of me talking to him which I don't. we have spoke on it but nothing major.. Anyway he's fed up with me being friends with my Bestfriend. and he say its either her or him. For 1. is this an ultimatum? and 2. Is it right? Am I wrong for not choosing??