What do u do if ur boifriend thinks ur cheatin on him with someone of the same ?

my boifriend is a little mad at me rite now because he thinks im cheatin on him wit my best friend who happens to be bi and i used to think i was that way too and he knows that and i cant get him over that insecurtie that i am not datin her and never will cus i dont cheat and if i did for somereason it sure as hell wouldnt be a chik ive tried explaining to him that i dont like chiks that way and that i love him way to much to do that to him i just dont know what to do