Like many other have said on here I have no issues with calling someone by their prefered pronouns once I have been made aware of this preference.
However I'm human so I make mistakes, so if I have known Jill as Bob for 10 years, chances are I may slip up one day, it won't be done intentionally to hurt or offend. But the level of venom that is directed at people who make a mistake is ridiculous.
And I agree with @CoolJazz, that it hits the raw nerve of the cognitive dissonance, that they are trying hard to mask with their 'identity'.
I have a name that is traditionally seen as male/masculine in Europe, as it happens in my job I work with a lot of European teams much of our communication will be via email at first with phone calls etc later. Invariably the assumption is made that I am male and has lead to some very funny/confused conversations.
I have several colleagues who despite having spend time with them face to face still slip up and refer to me as he/him, so what, it's a mistake, gently remind and move on.
However it seems more and more that instead of assuming that someone has made an honest error that they are being deliberately offensive .
As for: If said about a trans woman that's 3 times the existence of one of the most important features of their life has casually been denied, as I see it women are been told to deny that biology is important, but it is my biology, my breasts and my vagina that have caused me to be taken less seriously by multiple HCP's (even when I was a HCP myself)
It was my breasts that were been poked, groped and shouted at from the age of 11.
I have been expected to allow men access to my vagina because they bought dinner, they're a nice guy etc etc etc.
When attending multiple conference with a junior male member of staff, it has been assumed that he is my manager 'cause I'm the one with the vagina, he has the penis, even by people who knew I held the most senior position in the department.
In short we are been told that a biological fact for 51% of the population should be denied for a tiny proportion because to not do so is denying the existence of a feature of their lives. . .
Really................