hey there, havnt been on in ages (in the forums at least) and i had some assignments to do but im lazy so i figured ill randomly get on the forums and see whats up. gonna throw in my own words here:
1) Sex is predefined before birth
not sure what you mean by predefined, but anyway, when doctors assign a sex to a baby, they do so by a long table of criteria they have to meet. if all the female side of the table is checked, the baby is female, same for male. if there is even just one single criteria the baby doesnt meet or meets one that is in the other side of the table, the baby is considered intersexual. the list is incredibly specific and for example talks about the length of the penis of the baby (im not sure if i rememer correctly as if i havnt studied this in a while, but i think that under 3mm it is considered a female). funnily, if you were born with everything male but you were a mm short, you are considered intersexual, even if you grew up to have an average sized penis and seem to be similar to all the other guys.
assigning sex isnt as simple as many people want it to be, and just because it seems simple to you, doesnt mean it is. just because you are a person who is extremely comfortable with the typical identity that is accepted widely by society, doesnt mean that identity is the correct objective identity that other people with a penis should have.
this response is also relevant to your number 2.
3) One's gender must be inline with one's sex, one cannot contradict the other without disrupting the natural order
that is just plain wrong. gender is extremely fluid and changes from person to person, from culture to culture, and from time to time. in fact, the definition of the word itself describes just how subjective and cultural it is.
plus, nobody can say what the natural order is. it is some idealistic term that is used by people who want to build their ideas and opinions as based on science and biology but have no idea how science and biology work.
oversimplifying nature to just reproduction is incredibly arrogant and would probably never be done by any scientist that respects their field. first of all, gender identity doesnt even have to interfere with reproduction. if a guy identifies himself as a lesbian and doesnt feel a need to change their body, they can reproduce with even the woman they love. if its sexuality that you are trying to criticize here, it has been seen countless of times in countless of species. it is very obvious that homosexuality is definitely natural.
if it isnt natural what is it? unnatural? humans created it? humans are participating in it despite not actually wanting it? or is it supernatural and some sort of super power?
the whole "natural" argument is incredibly irrelevant to the topic in itself.
god is also a very weak argument here. not everybody believes in god, you cant prove god (despite of what you think), and even some people who believe in god (like me) would easily disagree with you on your views.
your gender identity (especially if its the typical identity that was established by your culture) is heavily influence by other people and your society, and if you want to call something unnatural, you might as well call all gender identities unnatural (including yours) as they would have been incredibly different in different contexts. what we see as feminine and masculine is heavily influenced by education, language, culture, tastes etc. etc. and the people who identify their gender as "opposite" to their sex, usually have the other typical gender identity which is accepted by their culture. in the end, those people are influenced by their culture just as much as you, they just happened to have a different taste or a different body.
sorry if this thread already reached a conclusion and i just posted an irrelevant wall of texts. good day.