If you accept the existence of brain sex, you accept the existence of intermediate brain sex, which means you theoretically accept the existence of intermediate gender identity. The difficulty in identifying such individuals (and in their self-identification) is that there are essentially no role models for them in society. Transsexuals and transvestites are openly ridiculed, the intersexed have been made invisible, when not surgically mutilated into 'normality,' and the people who have society's blessing to play with gender invariably turn out to strongly identify with one or the other standard gender identity (as it was acceptable for Al Jolson to appear in blackface, while actual black people were kept off-camera). Everything in human society pushes such people toward choosing to be either male or female. This is the emphasis even in transsexual circles.