First off, thank you for taking the time to craft a thoughtful reply, I genuinely appreciate it. It's such a rare trait these days, on any side of any debate.
That said, I feel like some of your own points want to ask some of your other points some questions.
"But if you're alone in a bathroom with one or the other, then your odds of being alone in the bathroom with one of them are 100%" this does make sense by itself. But I don't think it disproves my point. Having an actual cis man in the bathroom would make most women more nervous. This is important because it ties to my second point, which I think you misunderstood.
So you have mentioned above how 'trans women have male pattern criminality', and yet you also say this - "If a trans woman passes, they almost certainly already use the women's bathroom without causing any issues." Which, for the record, I totally agree with. I am sure many of us have been in bathrooms next to trans women and have not noticed. So..... what gives? Should we be retroactively horrified? Where was the criminality? If men and women are so deeply different and we simply cannot allow a trans woman to use a 'women's space' - what of all those women who pass and do so, as you yourself point out, 'with no issue'? Doesn't that pretty clearly show it's fine if they are there?
'Ah yes,' you might say, 'but it is the OTHER kind of trans women who are the issue, the kind with the male pattern criminality!' And here we get into the issue of now being 'kinds' of trans women? Delineated by... how well they pass? Why would that be, if a 'man cannot change his essence' - how are these very unique trans women managing to pee next to me in peace? Would a trans woman passing less well be a trans woman who poses more danger to me? ....why??
Then we come to your point of 'oh, trans men don't want to be in locker rooms with cis men'. Why do you think that is? Because they don't feel they belong there, or because they feel those cis men might be hostile to them? What point does that possibly prove other than 'nobody wants to share enclosed spaces with cis men, because they tend to be the most molesty group of people out there'? I can easily imagine a group of gay men also not wanting to share a locker room with cis men, but I would not think that makes gay men 'not really men'.
Thennnn we come to your point of 'oh I don't mind a trans man in MY bathroom, the same way I don't mind a woman in my bathroom, because they do not intimidate me'. I agree. That makes sense. But my point was not about trans men in MENS rooms. My point was about trans men in WOMEN's rooms.
Because the plan of everyone using bathrooms according to birth-genitals means the hurliest burliest beardiest swolest trans man - and there are some pretty impressive specimens out there - gets to be in MY bathroom. Do you see what that does? A guy with a neck beard and arms the thickness of my thigh can come into MY bathroom. And if I go 'Sir this is the ladies' room', he can go 'oh you see I was born with a vagina.'
He can just say that. Do I know if he was really born a woman? Who is going to be checking? Do you get it? If everyone born female can go into a women's restroom, that means that ANY MAN AT ALL can go into ANY WOMAN'S RESTROOM, just as he is, and simply say 'oh I was born with a vagina. This is all from T shots'. You, you personally, could go into a women's bathroom, look the woman there in the eye and say 'yeah, I know it looks weird, but I was actually born a woman', and there you are, a CIS man, in the women's bathroom. Do you get why that is worse than what we had before?
"And more to the point, I've yet to hear *anybody* (and I've ben asking for at least five years now) explain in any kind of verifiable or objective way, what the difference between a trans woman and a man even is!" So on these questions I do agree it is difficult to define things in 'verifiable and objective ways'. It is. Because human beings are really complex and weird. I can try to do so in a roundabout way.
It is my personal belief, from everything I have seen, read and heard on the subject, that male/female is not so much a dichotomy as a spectrum. And yes, 98% of the people will fall relatively clearly on one of the two 'sides'. But that doesn't make it not a spectrum, because you have so many people who clearly don't fit either box neatly. Start just with the recent surge in massive bulky 'muscle mommy' type women in sports (think Ilona Maher for example, or Natasha Aughey, or Alicia Napoleon. Of course these women are women. But we can definitely see that Ilona Maher is, let's say, a different type of woman than, say, Ariana Grande. Women can have different amounts of muscle mass, testosterone, ambition, aggression, whatever. So can men.
Then you come to the cases where the body is clearly confused. You have people born with both sets of genitals. Before, doctors used to just let the parents choose which set to remove, and would raise the child in whatever gender the parents had picked. If that kid grew up to feel differently gendered, would we be surprised? Then we have weird mixes - male chromosomes with female genitals. Female genitals outside but undescended testes inside the body. Is it weird if these people grow up to not feel entirely the gender they are being raised as? Not really. They are the middle ground. Somewhere in between.
My belief is that genitals alone do not make a man. If you god forbid lost yours, you would not stop being a man. If someone had surgically removed them from you at birth, you would also not stop being a man. We all have our spot on the spectrum, that has to do with our body but also our hormones, our brain chemistry, our life experiences.
Some of the differences between the sexes ARE biological - but biology is not fond of strict binaries. We see that in our vast differences. We have very effeminate women, to kind of medium-feminine women, to clearly pretty darn butch women, to very androgynous people, to fairly effeminate men, to regular degree sort of masculine men to extremely hyper masculine men, it's much more than just 'box a and box b'.
Then of course we come to the fact that one thing that happens very often when we start introducing these firm rules about 'women's spaces' is that masculine seeming women - muscular, sporty, short haired type women - tend to start getting harassed about being 'in the wrong bathroom' and suspected of being trans. How would we protect them from this? Do they need to wear transparent pants? Do they not get the same consideration other women get, because they're 'performing their gender wrong'? Because I have heard zero personal experiences from my circle of friends saying a trans woman assaulted them in a bathroom, but I have heard stories from women friends who have been questioned over their gender in a women's bathroom. Even had security called on them. For not 'looking sufficiently feminine'.
I'm pretty sure you at least heard of the case of Imane Khelif, the women's boxing gold medalist at the last Olympics. There was a literal witch hunt after her because she 'looked too manly' to random people. Trying to police allowed levels of 'manliness' in women is not something I will ever be able to get behind, nor would I ever consider it 'protecting women'.
Anyway, very long story short, I personally would be fine with 100% unisex bathrooms, have worked in spaces that had unisex bathrooms, and have had no issues using them. But I am very much against random citizens gatekeeping other random citizens from spaces they feel 'belong to them more', on what are clearly pretty shaky pretenses.