Maya, found out that I wasn't born female, would she be free to be rude to me without my having the redress to my position being untenable because she had a protected belief?
PPs have explained the difference between holding a belief and the manifestation of a belief. There are several examples upthread that discuss that nobody with a protected characteristic has the right to harass others on the basis of a protected belief or characteristic - and equally none may be harassed for not holding that belief or characteristic.
As others indicate, it might become an open question as to some people's threshold for 'rude' (was Mrs B in Miller's case?) or harassment. (An issue that was recognised as early as McPherson.)
Are you apprehensive that a workplace that abides by people's preferences might, in some undefined circumstances that you deem to be irrelevant, withhold the use of preferences when you'd rather that workplace is compelled to use them? - Even when they have no material reality?