The thing is that sexism is so baked into our world that it's virtually invisible. It's astonishing when you start to notice how often when someone says 'people', they actually mean 'men'.
A million tiny shades of subtle sexism - historic and present - pervade every situation.
This is why women are an invisible group whose thoughts are easily discounted, whose feelings don't matter, whose experiences are brushed aside. In order that we can centre the really important issues; those of men.
So our feelings that ridicule of our body parts ('knockers') is unpleasant, offensive, hurtful, all get brushed aside with 'it's a laugh, lighten up'. But a gender non-conforming male is so worried about his ego getting damaged by someone not recognising his invisible innate gender identity that we are expected to embrace his entry into women's single sex spaces - not only embrace, we're to celebrate it.