> But how can anyone _possibly_ deny the first part? How could anyone contradict an individual who states their own gender - on what grounds can you claim to know how someone feels about themselves better than they themself do?

A lot of us remember similar mistakes from their own teenage years. I had a short phase where I thought I was gay, because my classmates were starting to date girls but I had a close guy friend and wasn't very interested in girls. Then I looked it up, found some resources clearly explaining that's not what being gay is, and walked away more secure in my identity. If someone had told me that I couldn't be wrong, because nobody knows how I feel better than I do, I would probably have come out and then been confused and insecure for years.