I don't agree either. I was a tomboy and no one minded or made life difficult for me because of it. I remember my headteacher when I was about 7, showing off an airfix kit I had made to the whole school and saying how great it was that a girl had done it (a male headteacher too). I got mistaken for a boy, but when people realised I wasn't they apologised and that was that.
I was a teenage in the 80s when gender non-conformity was everywhere in the public eye and was considered cool. I'm straight, but had short hair and didn't wear make-up - still got boyfriends.
There are still plenty of GNC people including feminine gay men, masculine lesbians and just people who resist or mix up stereotypes to a greater or lesser extent, normal people and famous people too.
I do agree that gender stereotypes have got worse and more polarised since the 80s, but it's absolutely not true that you have to adhere to them. Of course if you're raised in a very sexist household and put under pressure then it's harder, but people still break free.
I get so annoyed by this massive pretence that everyone who'd not trans is living in some kind of pink and blue binary gender world, they're not. Sex is binary, that's our bodies and what they do. The rest is personality and culture and everyone's different.