Social constructs, culture and society
I have no dispute that gender is something which can only be observed within a cultural context and therefore has no fixed frame of reference but just because the frame of reference changes depending upon the society does not mean that an observation has no validity within that society. Therefore gender, which can only be interpreted within a society, nevertheless can have a meaning.
The same applies to analogies like beauty, politeness, violence, freedom, etc. Indeed, problems arise when people from one culture try to judge another culture by the attributes from their own frame of reference.
It is possible that someone who considers themselves transgender (or beautiful, or peaceful, or polite) in one culture might not if they put themselves in another culture. This does not invalidate the meaning of gender, it simply recognises that as a social construct, it necessarily must have a cultural frame of reference.
If we accept the necessity of having a cultural frame of reference for such matters, a question then arises how large or pervasive that culture is within the society in which it exists. The understanding of matters such as gender, violent behaviour and laziness are fairly uniform throughout the society in which they exist, perhaps because they directly impact upon the survival and growth of the society. The less that a society needs to concentrate on survival, the more open it can be to individual aberration (I am not using that word in any negative context). The more connected the aberrant individuals are, the more they are able to create a subculture within the society.
The larger the subculture, the more acceptable it becomes and the greater will be its chances of influencing the majority culture. (It is hard to imagine ‘50 shades of grey’ being acceptable, let alone so popular, if it were not preceded by 50 years of BDSM subculture and 40 years of sexual ‘liberation'.)
There can be little doubt that ‘transgender’ is a subculture rather than a definition of somebody living other than their chromosomes would indicate. (I know there are some who have crossed genders who are not part of the subculture, they are simply people who are living with the opposite gender to their original sex.) If all transgender male-to-females were recognisably female, and all transgender female-to-males were recognisably male, their could be little discussion or knowledge of them. There could be no question of which sex’s facilities to use because nobody could tell. There would not be a ‘T’ in LGBT because there would be no need (beyond some few legal ramifications which can be circumvented).
If we allow that people can identify as whatever they like (there are those who identify as a canine, and those who identify as almost anything else) and leave them to their relatively harmless self-identities, then the question becomes “to what extent will we allow other people’s delusions or inclinations to impact upon society?”
And that is a far wider issue than just talking about any one small portion of society.