simplistic understanding of sex and gender
The thing is hexed, I think it's who has the simplistic understanding, because you can't stop conflating sex and gender when they're different things.
Gender is behaviour, personality, cultural expression, stereotypes assigned to the sexes by society. We know this because gender stereotypes and roles are generally different in different societies and different times in history. There's nothing innate about them - they are just behaviours that society decides are masculine or feminine.
I accept that to some extent gender roles may be associated with biology, eg men being expected to show strength and domination and woman weakness and deference. But they are a still just roles. A woman can wear a suit, have short hair, enjoy boxing and work as a mechanic, and despited these things being generally "gendered" male, is still a woman.
You talk as if "experimenting with gender" somehow means you are a man or a woman based on your behaviours and choices. Why can't you just be the sex you are, and do what you like? Sex still exists, whatever you do. And sex is important because it has biological implications that means the different sexes need different kinds of support, healthcare, safety protections etc. - especially women because of certain biological disadvantages they have - namely being on average physically weaker and being prone to carrying and birthing babies.
Of course it's possible to be trans and wish you were / identify with / try to emulate the opposite sex, but you can only do that via stereotypical gender expression and hormones and surgery to imitate the opposite sex. Many trans people do this because it helps them, while understanding fully what sex they are.
If you could actually change sex using these measures, we would expect trans people in general to be like their "chosen" sex in measurable terms - number of pregnancies per 1000 women, for example. But they are not. They cannot be the opposite sex.
So what GC people favour is being able to express yourself in whatever gender expression you like, while helping everyone to understand you can't change sex. That is what the girls in my OP seemed to be doing - while not proclaiming themselves GC feminists, just because it's what seems to make sense to them – and I saw that as hopeful.
Whereas you seem to have a thought process that goes: 1) experiment with gender ----> 2) become the sex that is stereotypically associated with your chosen gender expression.
Can you really not see that that is hugely sexist, because it tells people they're only a girl if they do and only a boy if they do ?
And that is is also harmful, because it encourages kids to think they can actually be the opposite sex – when in fact going down that road could well lead them to a sterile limbo lad where they will have their biological sex characteristics hugely damaged but will not actually gain those of the opposite sex?
When you say gender experimentation is fine - yes it is but saying you are a "he" when you are a girl is not gender experimentation, it's just misleading, harmful sex mislabelling. Why can't a girl just be a girl who chooses "masculine" self-expression? Why must she therefore become a boy?