[..]
I don't understand how it's Sexist? Am I being thick?
[..]
Reality doesn't matter, only the accusation. That's the way it works. No doubt it's racist too. Probably "transphobic" as well.
Some people like to attach false authority to their opinion. "I don't like it" doesn't do that. "It's <insert currently unfashionable prejudice here>" does. It changes the statement from "I think this is bad" to "this is bad and if you don't agree you are bad too".