Biologically, you are a male or a female. Anything else is a birth defect typically brought about by abnormalities in sex chromosomes. A birth defect is not classified as a new gender. In fact, those with birth defects are still either male or female, but with complications in their genetic code that lead to a deluge of health issues. Humans are sexually dimorphic (not to be confused with
sexuality), and there are only two main genders. There may also be a few edge cases also related to genetic defects that may garner a different classification, but those are extremely rare medical conditions.
(You can read about some of these chromosome issues here:
https://en.wikipedia.org/wiki/Sex_chromosome_disorders)
Transgenderism is when a person has gender dysphoria. If you do not have gender dysphoria, you are not trans. People who claim to be trans without gender dysphoria or self-diagnose are known as
trans-trenders. You might find people like this on Tumblr, and it's these people that are often thrust into the limelight by the "anti-SJW" crowd. This -- Tumblr and similar sites -- is where the vast majority of the
absurdly growing list of fake genders comes from. If you are actually trans, it means your brain chemistry and body are mismatched. Your brain might operate like a woman's but you were born a male. This leads to an unhealthy reaction from the brain that results in gender dysphoria. These people may choose to transition to the opposite sex in order to relieve their brains from the body mismatch and in doing so, bring their brain activity under control. Keep in mind, transgender folks are still one or the other -- either male or female. There are no other sexes/genders and transitions occur from one to the other.
So why do people come up with all these genders? That's where
gender identity and
expression comes into the fold. This is where a man might act in a way society traditionally prescribes women to act or vice versa. In the past, these people were known as "tom boys" or "tom girls." In some circles, they might even be social outcasts for going against the traditional status quo. It is my personal belief that a person can act however they please so long as nobody else's rights are violated, which means dress how you like, speak how you like, act how you like, be what you want to be. Nonetheless, the laundry list of infinite genders is not a substantiated concept. The "gender spectrum" you may hear about just describes how "manly" or "womanly" you act according to society's current standards. The "gender spectrum" or laundry list of genders you see is an attempt to classify and place labels on the different way people express themselves in relation to how society wishes they express themselves. This is where you derive the "gender is a social construct" idea. Nonetheless, a person's actions do not change the fact that biologically they are either a man or a woman, and that fact is immutable. In addition, our biology does influence our brains. Male and female brains are wired differently, and this brings about certain behavioral differences between males and females. Note that this is generalized, and that people's brains may vary greatly. Think of it was two bell curves -- one for males, one for females -- with a valley in between them. There is a "male" brain with standard deviations away from the "standard" male brain, and a similar bell curve for females. Transgender people fit on these bell curves differently, of course. Biologically, you are male or female but your brain is far more complex with far more variation. This in turn influences behavior and by extension, gender identity and expression. Gender identity/expression is just another word for variations in behavior, except based around a comparative social context.
Then why do people do it? Why create all these genders? Here's how I understand it, and you are welcome to debate my interpretation:
For some, it's an effort to be included, to impose a meaning over one's behavior. Let's use an example. Some females act very masculine and are interested in things typically thought to be the domain of "men." In an effort to affirm them and their actions, they create new identifiers for how they person behaves. Humans have an ingrained want to belong. Belonging is the third tier on
Maslow's Hierarchy of Needs, right above basic needs and shelter. By placing a label on their behavior, they're given an identity that more closely matches what they are. They don't fit the stereotypical female identity so they assume a new identity to fulfill this need to belong.
For others, it's a way to seek attention. This may sound cruel, but it may also be true and is worth pointing out. People will go to great lengths to secure attention. This again inhabits the third level of Maslow's Hierarchy. For example,
Japanese teenage girls regularly enter prostitution while in high school because that's the only place they feel needed, if only to satisfy a grown man's lust. Or how about the observation that
babies develop growth problems and can even die without human touch? The need for attention, for belonging, is powerful. By creating new identities for themselves, by appearing unique and different, one can attempt to garner more attention. Attention-seeking may be seen as negative by society, but it's a psychological fact that humans need attention. Attention likely plays a large role in the so-called "Tumblr-culture" of "infinite genders."
That's my take. I hope whoever is reading this gained a new perspective. Feel free to respond! I'd love to have a good-natured conversation with someone.