I think a basic problem is that there are four underlying polarities. Biologic sex, which is quite binary with only a few exceptions that basically prove the rule (XXY in mammals for example). Sexual preference (who you want to have sex with) which as Kinsey showed is much less binary. Social functioning on a "stereotypical" masculine / feminine scale, a concept that we do not have a decent word for but basically means that we posit that male and female behavior admits to a binary polarity. But clearly individuals do not. For most mammals this polarity has considerable truth. Humans are very atypical mammals and no one fits the extreme, in fact one who did would seem quite pathological. The most "masculine" man will nurture children and the most "feminine" woman run marathons. Lastly there is the desire to fit into a gender "role". The vast majority of humans have no issue here, partly I think because social functioning (and sexual preference) are not binary. A lesbian weightlifter can "identify" as a woman and a stay at home dad can "identify' as a man with out any deep "gender dysphoria". All these distinctions have some biological input, women with high testosterone levels are more likely to be athletic,"masculine" and lesbian. but all except biological sex have social input too. Trans is different. Why trans individuals are not comfortable fitting into one of these flexible roles is not clear.