Something from within the last few decades isn’t really archaic, that is generally reserved for (well?) over a hundred years old or older, and the vast majority of Lemmy users are either North American or European. Anglo/Eurocentric is going to be the relative norm on social media in general outside of specific apps, and those then trend East/South East Asiacentric due to their development origin. You should not be surprised to encounter this.

Heteronormative will also currently still trend as a default since over 80% of the population identifies as such. Intersex is also somewhere around or under 1% of the population. While gender and sex can most certainly be different, at least currently the supermajority of people will have these aligned and will use them interchangeably. This shouldn’t invalidate or be used to discriminate against those that aren’t heteronormative by any means, but something that is true 80-90% of the time falls within the colloquial or layman’s qualifications for a broad assumption of “how the world works”.

The fact that intersex people get to decide their primary sex (or more likely had a doctor decide for them at birth) on government forms is somewhat analogous to 3 wheeled motor vehicles that can be registered as either a car or a motorcycle depending on the State and/or county. This does not invalidate car or motorcycle as categories, nor does it invalidate andly other means of transport.