I'm sorry, but if you believe the fundamental difference between men and women isn't simply that we are different sexes, but that our minds are different, so different in fact that if a male person thinks "like a woman" then he actually is a woman, and vice versa...
... and if you believe that female people, the half of humanity who have been disempowered, objectified, sexualised, abused, exploited and marginalised all over the world and through most of human history simply because of our bodies are so unimportant that we don't even justify having our own name, let alone legal rights and protections, but a man who wants to take us as a metaphor to express his own sense of self does...
then yes you are sexist.
You might not like that, and I'm sorry about that. But maybe, just being honest about what someone saying they are "trans" is really saying about who they believe men and women are and the limitations they believe have to come along with those labels not just for themselves but for everyone else as well is worth thinking about.
Because being gay is so everyday it's not even worth noticing.
And being gender non-conforming is great - as a Feminist I'm all for smashing those stereotypes.
But believing that if a boy or a girl, or a man or a woman, doesn't think in a gender-conforming way then they must be the actual opposite sex on the inside - well that is just a very sad and sexist way to think, and the sooner people realise that and start challenging it the better.