You're really only COSMETICALLY female. It's not like you actually chose to be a woman.
I think this a very simple way of expressing a truth about how we as a society construct identity. Taking nationalities as an example, immigrants are often described as "not real Australians" in contrast to the "real Australians" being those who were born here. However the immigrant made the decision to live here, they put in the effort to move, so it seems strange to privilege the identity that only exists by happen stance as the real one.
Obviously this applies to the denigration of trans identities; we are "not real women", while someone who never gave their gender a second thought is (I managed to get that the first time I told someone that I was trans; I had been out to them less than five minutes and they said I cannot be a real woman :/ ).
I also think that it may apply to us child-free by choice people as a desire for, or having children, is seen as a requirement to be a real person, or for it to be a real relationship. It probably applies more to women, there seems to be a greater stigma to being a childless woman.
Anyway, it seems strange that we consider trans/immigrant/child-free identities as less real, as these people have actually given some thought to these aspects of their lives. If you can bumble your way through life without giving an aspect of your identity a second thought, I don't think there are any grounds to say that identity is more real than that of someone who gave that aspect some thought.
Better yet lets not judge the realness of other people's identities.
