Is gender reassignment a function of sexism?

As a preamble, I apologize to anyone who is offended by this because they have had, are having, want to have gender reassignment or for any other reason. I am not transphobic, at least I don't think I am.

My question has to do with the basis for wanting to be reassigned.

Transpeople say that they always knew that they had a gender identity that did not match their physique.

To me, this seems to be an issue of not conforming to gender roles; that if you want to do mannish/womanish things then you ought to be a man/woman externally.

Isn't that sexism? Isn't that, in a very real way, obeying old-fashioned binary gender norms?

What's wrong with being as femme or manly as you like, wearing what you like, calling yourself he or she as you like? NOTHING!

Why take hormones and/or surgeries that will probably impair your sensation? If ritual circumcision is wrong because it cuts healthy flesh, why would you cut your healthy flesh? To fit into 1950s gender norms?