Actually, the duck example is supposed to highlight the importance of pragmatism and common sense. In normal social situations, where people are fully clothed, there is no reason why a trans woman can't 'function' in the same way a biological woman would. Of course, things would be different in locker rooms or spas, for example. Therefore, my argument is that the context is important, and we should trust the common sense of pragmatic people rather than ideologues on both sides as to how trans people should be best accommodated.