Well, mostly you’re just off-topic, but it’s my fault. It honestly didn’t occur to me that you would pursue the discussion by doubling down on being rude and tactless. However, for the sake of any woman who doesn’t escape in time: the reasons normal people don’t try to discuss pregnancy with a pregnant person unless she brings it up are (1) if she’s far enough along that you think the pregnancy is obvious, she’s far enough along to be sick of the topic; (2) sharp as you are, lots of sharp people have guessed wrong about someone being pregnant; (3) pregnancy is often a source of stress and anxiety for the woman so it is polite to wait for her to begin any talk about it, and be alert for any signal that she wants to change the subject; (4) many women continue to look obviously pregnant for a while after an abortion or miscarriage.
Not sure I even believe you, but (1) yeah, incredibly boorish; but (2) at least slightly more analogous in the sense that it’s something the person’s more recent acquaintances may not know about, and noising it about makes you a horse’s ass.
Of course, mostly, you’re just off-topic: we’re talking about what you say to other people, or allow them to overhear, more than about what you say in private to a person you think is transgender, because while the latter is apt to be hurtful and rude, the former is much worse.
Here I don’t believe you at all, but that’s okay. First, off, it’s not obvious to everyone. Not to the person’s recent personal acquaintances, not to those with whom she or he interacts less often or at a distance, and not to those with whom the person has decided not to share his or her status. Second, it’s rude because discussing people who are not present is rude. Third, the circumstances surrounding and leading to a person’s transgender status are liable to be unhappy, if not traumatic, and it is wrong to create a situation in which the person will certainly have to fend off more curiosity (well-meaning and not). Fourth, spreading the information increases the likelihood that it will come to the attention of people who will seek to harm the transgender person. This is not an inconsiderable risk (cite is in your native Australian for your convenience). Even if violence does not occur, any number of other kinds of discrimination, large and small, are suddenly a greater danger.
As long as you hang on to the idea that it is not cool, and not okay, to contradict a person’s chosen, legal identity, and act accordingly, and err on the side of quietude whenever in doubt, you probably won’t do any harm. But strive not to draw analogies and think in terms that basically just exist to construct a mental framework in which it’s okay to do bad things that might harm other people. For example, you dropped the Orwell analogy: positive step.