You have a penis or a vagina, indicating biological gender. But unless you get surgery (and even if you do), there’s still only two genders you are…male or female. That’s probably a very controversial statement, but I don’t understand how someone can not be male or female.

You probably understand the concept that a person can be "more masculine", or "less masculine", right? Or more feminine or less feminine?

Say a man might do a bunch of manly things and exhibit many manly traits. Be big and hairy, play sports, assertive and strong personality etc. etc.

Another man might not do any of those things, they might be sleek and slender, indulge in creative or nurturing pursuits, have a soft or effeminate voice and personality.

It's very easy to see that there aren't really "only two genders" but rather a sliding scale between the two. Thousands of different elements that can be associated with one of these "two sides" with a varying degree of accuracy. Feminine men exist and masculine women exist. To attempt to reduce the incredibly complex and varied spectrum of human gender expression down into 2 discreet categories is.. unhelpful, at best. Even categorizing these things as "masculine" and "feminine" isn't really ideal, but we're kind of stuck dealing with a lot of baggage from our past.

Society tends to look positively on people who conform to the social gender norms, such as the first man in this example here. It tends to be judgemental of those who don't, regarding them as somehow lesser than those who "fit the mold" so to speak.

Non-binary people typically are just people who don't want to associate with either gender. They want to be able to be free to express and behave as an individual without being beholden to gender norms, without being judged for the fact that they don't fit into societies box.

Part of why people identify as "non binary" is to wrap their head around their own internal shit. No matter how much you like doing things that are "unmanly", most men were born and raised to be men and still feel that pressure to conform. By having a different identity it's easier to accept yourself and your own desires rather than inadvertently caving to social pressures and constantly feel like being you is doing something "wrong".

"Non-binary" can mean a lot of things, but at the end of the day most people who self identify as "non-binary" are more or less saying they want to "opt-out" of this system or structure through which people are categorized and judged.