Biological, traditional, reasons of political opportunism, bringing science in to explain inexplicable...
Worse, lawyers deciding what is the appropriate language. Take some text written by lawyers and see what kind of freak language it is!
The problem here is change in social structure which now has very little if anything to do with biological.
Religious reasons are totally outdated and calling upon Bible will soon be outlawed and punishable. And it should be so.
Even Plato predicted this, though at those times it was not really possible to apply. Church did have some control of the part of children population
in times where there was so much calamity and destruction, from wars, brutal lords or plague - seminaries etc. Then, it was the school system which
isolated children and the role of family was diminished in favor of the social differentiation. School and preparation for particular function in
society was more important than family membership.
Now we have digitized world with its own needs which will by far outweigh all other needs. The need to process formal physical connection of children
with their closest care givers is predominant. Gender doesn't matter at all. This is not about giving rights to the gay population, they may just fit
nicely into it, but are not the cause for this measure.
The attack on the language protocols is also for the same reason. This is more visible in English (the youngest and most modern language of all),
because there already are words which don't precise gender - like "friend" or "relative" (there is no friend-in or colleague-in), and many family
positions don't exist in English "son in law, father in law" etc, those are not "words", those are constructs. "Uncle" applies to both
father's and mother's brother, etc. This is also result of the bureaucratic needs to dominate and adapt all other spheres of human domain to its own
This kind of language adaptation is visible from Shakespeare times. Especially in his sonnets (God knows who actually wrote them, Pope?). Some of
those sonnets are introducing legal language, and they are mostly about how a man should create family so he can have a "heir". Pretty much
accurately anticipating Henry VIII little "problem"
The Age of Informatics probably has much more in stock for us. This is just the beginning. These changes are never to suit personal needs. They are
always to suit the needs of the latest technological phase of processing people in an impersonal way.