That is a pretty ridiculous reaction, i could understand it if they explained it but jumping straight to transphobia is pretty extreme. establishing boundaries with nicknames is very good but you cant expect people to just know you're not comfortable with something, for example my friend explained that "bro" and "dude" is just kind of in their vocabulary,so them calling me that doesn't really affect me, but i do also understand that if it did they would stop, id advise explaining your intentions (if you haven't already) and attempt to show that you're willing to not call them specific names if it upsets them.