the notion that the signifiers of gender can only be identified by the individual? What if the sense of womanhood is felt differently, not only between a trans woman and a biological woman, but between two biological women?
Of course it is, because it's what we've been saying - it's personality. We are all different.
a world that still applies gender differentiation to almost every human experience
Does it? How does it? Is drinking a cup of tea different for me as a woman than it is for a man? Is this the 'men are from mars' school of thought? Because most feminists take issue with that.
Sexist stereotyping exists, of course. This doesn't actually mean that women are born liking the colour pink, or knitting, or kittens, or pillow fights.
what if someone finds an individual sense of belonging in a societal group that they are barred from, simply because of a physical attribute?
Like, say, older people who would really really like to discover their inner child and enroll in kindergarten?
(I refer you to StephonKnee Walsht, but do take care googling). Or an able bodied person who strongly identifies with amputees?
dailycaller.com/2015/06/02/meet-the-transabled-able-bodied-people-who-identity-as-disabled/
Are we to abandon all boundaries, or criteria, on the basis that some people will be very sad if they aren't allowed to join a particular group?