untrue to their own humanity
Conceptually-speaking, there is just no such thing as one's "own humanity". Might as well be speaking of fairies in the woods.

As for your query as to how we assess what is natural
Natural to the individual, as per your post.

As for your query as to how we assess what is natural, I would suggest looking to the original conditions in which humanity developed and the behaviours displayed in those times; and contrast that with how these behaviours changed when the individual became increasingly defined by the abstract creation that is society.
This is the critical error - there has been society as long as there have been humans. There is no separating the two, so attempts to divine a human nature 'beneath' society is akin to searching for an era in which the Earth was wholly flat.

What would your response be to all those examples of sexuality being down to social (and particularly artificial) conditioning?
'Little Johnny turned gay after the bad man taught him gayness' is easily dismissed with the far more reasonable inference that Johnny was gay all along.

Sexual orientation is emphatically not something that can be taught.