Teachers were forever having to chase me into the “correct" gender group; the boys got the more interesting toys and games. For years I thought of gender as entirely a social construct.
I ran into this on another site, but it rang true to me: There are two kinds of people who don't grok transsexuality: those with a very strong attachment to their assigned gender and those with no attachment to their assigned gender.
I know there are people for whom gender is an intrinsic part of their identity. There are people willing to risk death to be true to their gender. I refer to myself as “she" because I have an innie. If I had an outie, I'd refer to myself as “he" but I'd be “me" in either case. It's like blue to someone who's color blind. You may experience it as cobalt or indigo, to me it's gray.