"For me, it only "clicked" once I understood that gender and sexuality are completely arbitrary and subjective social constructs that try to describe a whole spectrum of multidimensional behaviors and (potentially repressed) feelings, so there's little point in trying to objectively categorize them - it's all about the subjective impression of the person themself"

Sex is a fundamental property of living organisms; it is not determined by thoughts or feelings.

The argument about gender (and pronouns) being an arbitrary social construct does not imply that it's the individual's unilateral choice - the fact that naming people and referring to them is a social construct means that the social consensus determines how people will be addressed and the individual does not get a veto vote - e.g. if someone asserts that their identity requires them to be called Your Majesty, the society will simply ignore that demand. So the demand for non-standard pronouns essentially is up to the society; someone may want others to use e.g. xe/xir, but it does not necessarily mean that this desire has to be honored, that is an arbitrary social construct which can plausibly be different in different subcommunities; in some communities these pronouns fit the social construct and it's mandatory to use them, in other communities it goes against the social construct and it's considered unreasonable to demand that they get used.

There are many other parallels - e.g. the criteria for using (and expecting/demanding of use) of formal vs informal "You" in many languages, the expectation on how mandatory it is to use specific prefixes or honorifics (Sir/Ms/Dr), etc; and in all those cases it's an arbitrary social construct and the wishes of the individual can be and are shunned whenever they go beyond what the locally prevailing social norms require.