1) Can GC explain what is wrong with what I said above? Can you break what I said down and tell me what's wrong with what I said above and why you disagree with it.

"Gender identity is a psychological thing." "They feel like a man and other they feel like a woman." "They feel they are in the wrong body." "[...]who identifies as a man. [...] who identifies as a woman."

Feelings, thoughts, belief and psychological experience are not reality. For example, schizophrenic people think that their hallucination is real, but, in fact, it's not real. You can believe that earth is flat, still doesn't change the fact that it isn't. Claustrophobic people experience intense fear when being in small spaces, doesn't mean small spaces dangerous. Senior citizens can feel young, doesn't change the fact that they're old. Teens feel like they know everything, doesn't mean they does. You can identify as a cat and speak in meows, still doesn't make you a cat.

You can feel and identify as whatever, but the reality won't change.

2) Why do you say sex is not assigned at birth? If sex is not assigned at birth then what is it?

As other commenters mentioned: sex is determined in conception, observed and recorded at birth. Human have only two sexes, just like other mammalians: it's either male or female. Intersex people--which can look androgynous, have vague looking genitals and/or have secondary sex traits from the other sex--are either male or female depending on factors such as SRY gene, whether they produce sperm or ova, etc.

3) Why do you say people can't be born in the wrong body?

Are short people who think being tall is nice born in the wrong body? Were men who hate their receding hairline born in the wrong body? Were people with brown hair who wishes to have jet black hair born in the wrong body? Were squinty eyed people who wishes to have big eyes born in the wrong body? Were people with egg allergy who wishes to be able to food that contains eggs born in the wrong body? Were people who have congenital disorders born in the wrong body?

To me, all the questions above has only one answer: no, they weren't not born in the wrong body. Our bodies might have traits or conditions we don't desire. To accept, cope and make the best of it is a part of life.

Also, didn't the fishes take back "born in the wrong body" slogan?

4) Can you explain why gender identity does not exist? Don't cisgender people identify as/feel like they are the sex they were assigned at birth? Don't other animals identify as their sex assigned at birth due to lack of cognitive ability to identify as anything but their own sex?

What does "gender identity" means? AFAIK it's hardboiled gender stereotypes. Trans ID-ed males "feel" like a woman because they like dresses, want boobs and loves make-up. Trans ID-ed females "feel" like a man because they love sports, hates their boobs and prefer to mow the lawn to cleaning the house. Again, feeling doesn't equal reality.

Men never experienced what it's like to be a woman in any point of their lives, women never experienced what it's like to be a man. How do you know what it feels like to be someone you don't?

For example, every single adult were a toddler at one point of life. People who remember their toddler-hood know what it feels like to be a toddler because they naturally have experienced it. And yet, no adult can ID as a toddler, tell everybody they're a toddler, go to kindergarten to study with toddlers or compete in toddler's drawing competitions without the whole world think they're perverts, crazy or mentally ill. How can self-ID-ing yourself based on "feeling like someone you have never been before" be more legit than ID-ing as a toddler?

They mean a dog or a fish also have a gender identity and they can only identify as their own sex unlike humans...

I'm an adult human female who watch boxing championships, hate dresses, wish I were an A-cup (if not downright flat), don't enjoy periods, play video games catered to males and I don't own cosmetics. I don't do things those who "feel like a woman" think women do and do things that they think women don't. I don't "feel" like a woman, I don't "believe" I'm a woman, I just am. Before you assume dogs and fish have gender identity, please analyze mine because I (and the majority of human population) seem to be a walking contradiction to gender identity theory.

5) I can't wrap my head around what trans people mean with "valid." Oxford dictionary said that valid means "(of an argument or point) having a sound basis in logic or fact; reasonable or cogent." From what I can gather, transgenderism is not logical, factual, reasonable or cogent.

What the heck with lgbtphobic lol. I don't go to church, doesn't mean I'm Christian-phobic; I don't fast during Ramadan, doesn't mean I'm Islamophobic; I don't like touching any kind of reptiles, doesn't mean I have herpetophobia; I don't sleep with lesbians, doesn't mean I hate lesbians. You're entitled to whatever you believe, you're allowed to do to yourself whatever you want to; just don't expect other people to accept your belief or think that what you do is normal. After all, other people are entitled to their own beliefs.