It's my (extremely outside) understanding that the concepts of physical/genetic/choose-something-objective gender are not what people mean in these conversations - nor are they referring to the subjective identity of what kinds of people one feels sexually attracted to. They are referring to a third thing, also subjective, best described as "what gender you feel like", which is very much like how old or young you feel, except much more closely related to one's identity (I "feel" young, which is great, but I also "feel" male, which is more than great - it's critically important to my self identity). So, both of these things (age and gender) have subjective and objectives measures - the difference is that "years old", which term the law uses, is patently objective both legally and linguistically, whereas on the other hand "male/female/etc", which terms the law also uses, is less objective linguistically (as described above), and therefore there is at least one basis on which you could argue for a legal change of gender - a basis that does not exist for a change-of-age argument.