The thing is, there are assumptions made at every turn.
You might see a little boy with very long hair and a pink Tshirt and think "Pfft fucking smug Grauniad reader twats forcing their agenda on their child", where I might see a hand-me-down Tshirt from his big sister/cousin (a Tshirt is just a Tshirt after all) and maybe he just has really lovely hair and his parents don't want to cut it yet cos it looks flipping gorgeous. Or maybe he hates the hairdressers and it's too much hassle!
As I said before, my mother was one of those crunchy types and strongly resisted the tide of pink (70s/80s), and we weren't allowed 'girly' stuff, pink or Barbies. I leant more to the tomboy side anyway, DSis was more feminine, but DMs attitude left me feeling guilty for liking girly stuff so I resisted femininity even more. Took me til my 20s to accept pink is just a colour and a nice one as well. Daft.
Most little girls I know have had a very strong girly/pink phase from 4-8ish then grown out of it, and developed their own tastes and preferences more.
Moderation in all things.
We all exist on a spectrum of feminine/masculine and this can ebb and flow with lifestyle and social groups.
It genuinely is harmful to discourage children from playing with anything, whether people are being overly PC or sexist. Both extremes are damaging to the child and it does have a knock on effect.
Everydaysexism.com might be worth a gander OP, and a Google of 'toxic masculinity', as reinforcing current stereotypes for boys hurts both sexes.
Clothing, Little Miss books, and boy and girl aisles in toy shops have a drip drip drip effect, insignificant in themselves, but poisonous en masse.