system
Given the following post, respond as if you're commenting on Reddit:
human
CMV: I believe that being gender fluid is a choice and not something you are BORN as

I 100% believe in transgender people. I know their struggle.

However I don't think it is possible to be BORN as both genders.

I believe the people who say they are genderfluid are actually simply transgender. NOBODY wants to be transgender. Saying you are genderfluid is empowering and can make a transgender person feel better and more in control. Also being genderfluid is "fun" for some.

The people that have problems with their bodies and yet say they are gender fluid? I think they are just transgender in denial.

In order to be BORN as both genders, you would need two brains for that, which isn't possible.

There is no evidence of such a thing.

Lack of evidence doesn't mean proof of existence.

Otherwise, "otherkin" (like mentioned below) and any other idea would be considered real no matter what, because there is nothing that disproves it.

The brain develops with a set sex of male or female in the womb. The brains of males and females are physically different. This is an undeniable fact.

For "genderfluid" to be real, brain would need to go through a constant biological metamorphosis between male and female during the entire life of a person. This can not (and would not) happen once the brain has developed. Otherwise transgender people, MTF and FTM, would NOT exist. The basis of transgender concept is that gender can not be changed no matter what once the brain developed in the womb.

For "non-binary" to be real, there would need to exist another sex hormone that isn't estrogen nor testosterone. Emphasis in "sex hormone" medical term. This 3rd sexual hormone does not exist.

There is no 3rd sex hormone and its impossible for brain to constantly metamorphose once developed. Ergo, 1 brain sex that doesn't change.

The brain sex can only be physically male or female (there are no other choice outside of the estrogen and testosterone).

There is no 3rd option, not even for intersex people. Not even for transpeople. The sex hormones do not create a "brain that constantly changes physically" in neither case.

There is no precedent, evidence, antecedent of anything like a 3rd brain that is not male or female. (And no, an internet blog with the "experiences" of a teenager doesn't count).

Detractors may be skeptical, it may sound mean, it may make some internet blogs angry, but its a fact and shouldn't be hidden. That would only spread misinformation. 

There is no fact or study that proves "non-binary brain" or "genderfluid brain" are real, its just suppositions and speculation.

Science has proven differences between male and female brain, but not the existence of other kind of brains.
gpt
I think you and perhaps some of the people you have been talking to have a few misconceptions about what it means to be gender fluid.

People that identify as gender fluid don't say they were born as both boy and girl. They identify at some point in their lives that they feel themselves to be a dynamic mix of both girl and boy, some days feeling more girlish, some days feeling more boyish. 

Saying that someone who is gender fluid is probably transgendered is like saying someone who is bisexual is probably gay. It's an insult and very revealing of your lack of education in this area.

Gender fluidity isn't really a choice, nor is it something you're born with, exactly. Their brain and hormonal chemistry isn't really their choice. It is their choice to ask people to refer to them as gender fluid though, or to refer to themselves that way, and you should respect that.