Hate of trans people is certainly not coming from the US.

Why does it make the most sense? The word Geschlecht can mean both sex and gender.

No one cares, except for all the people who do.

Maybe the problem is that people don’t care and make uninformed statements?

Unless you consider intersex the third sex.

I’m not sure though what relevance the existence of two biological sexes has for those fans.

The same relevance it has for everyone saying there are only two genders: It’s part of their culture war.