Firstly, do you think I go around taunting people about their self-perceptions of their gender? Even if it wasn't something I could be fired for (which it is) I don't actually take delight in causing other people psychological distress. I am asking these questions here, in this forum, because I want to hear better reasons other than "it's the nice thing to do".
Secondly, I listed several cases where your rules of politeness don't apply and aren't the norm - how are those different? This whole "take people's identity claims without question" works in some limited contexts, but breaks down in so many different cases that it really isn't a generalizable moral principle. At best what you propose is a useful tactic for having pleasant water cooler conversations with strangers and acquaintances. It is not a solution for the rest of life where our identities have important implications for how we relate to each other.
Thirdly, that social etiquette is the only justification you provide suggests you understand our acceptance of these identity claims is merely a social fiction. Now, I am a well-socialized adult, which means I understand how often little fictions, little white-lies grease the gears of society. And like I said previously, I don't like to cause others undue distress. But I also think forcing other people to lie (by threatening their job, for example) to be dehumanizing and immoral in its own right.