@Gibbonsgibbonsgibbons
Because it's about trying to make people say the opposite thing to what they can see.
Babies are tricky to sex on sight unless you're changing their nappy.
Children are very often tricky to sex too - people are usually using sex stereotypes which is why my 10yr old DS who likes his hair long is nearly always referred to as a girl from behind.
Humans have evolved to be able to accurately sex adults nearly all the time & when we get it wrong it's usually corrected as soon as the person speaks or moves (extra data for our clever brains).
The people who want to pretend they are the other sex dislike that the truth is usually so obvious to us.
The event sounds a bit bonkers & if alcohol is involved at all there is no way that most people will have the capacity to fight a lifetime of using pronouns correctly!
All the trans folk that I know don't believe that they are the other sex. That's a bit of a lie. They believe that they're the other gender, assuming that their identity lies on the assumed gender binary.
Trans people are usually hyper aware that they aren't the sex that their gender is typically assumed to be. No trans woman thinks that they have XX chromosomes, nor do trans men believe that they have XY.
Additionally, misgendering people on accident; completely okay! Most trans people believe in self-identification, sure, but they also believe in gender performativity theory, which is that you are the gender that you perform daily, and if you misread their gender while drunk or unknowing, there's nothing to be ashamed of there.
The problem arises when you make a point of misgendering them repeatedly. This is demonstrably harmful, and even if you don't believe in their identity - which is fine, I can't make you believe in a philosophy that you haven't been convinced on - rejecting trans identity doubles their chance of suicide.
This study shows that the risk factor for suicdality literally doubles if they are rejected in different parts of their life. 6.3% -> 13.4% in rejection of their identity in employment/school/healthcare/housing. 5.1% -> 10.5% in cases of familial rejection. 6.3% -> 13.1% in cases of religion.
This one is an international study that... well, the results section of the study says it better than I could.
"Gender-based victimization, discrimination, bullying, violence, being rejected by the family, friends, and community; harassment by intimate partner, family members, police and public; discrimination and ill treatment at health-care system are the major risk factors that influence the suicidal behavior among transgender persons."
I will note that this has to do with discrimination, and not just rejecting their gender identity like you are. I'm not implying that you discriminate against them, it's just a case that every little helps, and acceptance is much more poignant than apathy, and being apathetic of their identity can come across as hateful to trans people who deal with discriminatory people daily.
If you would like me to explain why I believe that trans people's identities are 'valid', or would like to know anything else. Do ask! Inquiry is the soul of knowledge ^^ .