Why is it so hard to understand that gender isn't a binary?

Everyone else did a great job of explaining that gender is a social construct. And really, the gender binary feels safe--it's what people know and understand. To challenge this idea makes people question how they live, identify, and interact with others. Sometimes, that's too scary and people refuse to change. Change is definitely possible though! I know a lot of people who at first didn't understand people who were trans, genderqueer, etc, but they were open to learning. I think the way our culture is moving, at least in America, people are getting more exposed to the idea that gender isn't a binary, which is helping. Usually, there is pushback when that kind of social change happens, which is why there is still such a struggle for people to get past the binary. And hey, some people feel threatened by it--if someone benefits somehow from the gender binary, they aren't thrilled to have that idea challenged.

People don't like when others are different. They like things to be basic and restricted. Or they all have their own opinion.

It really depends on the culture you're from. In American society, we grow up with the typical thought of a gender binary, boys and girls. It's only once we get older that some people discover that gender isn't all black and white, or... Pink and blue, I suppose, and sometimes it's hard for people to get used to.

Many people haven't experienced not being m/f so they can't comprehend it. Though it puzzles me how many religious people can't understand because there is 'no proof' or they can't see it. Surely of all people they should be understanding of the indescribable.

People tend to fear the unknown. If it doesn't make sense to them they automatically assume that it is wrong. Anything that isn't considered normal by societies standards is usually rejected. This is why hopefully in the future we will have a more open minded and accepting environment. This way everyone will feel welcome despite what they choose to identify as! :)

We are raised, in western societies at least, to think that there is only two genders. Because people are trained into that, basically from birth, it is hard for them to change something that ingrained in their minds.

Because there isn't enough education on sex and gender and most importantly acceptance! I feel that the education system is very flawed and needs to catch up with todays society.

Some people don't understand that definitions of things can change, and then when you constantly hear things growing up, you tend to believe them. Odds are most people heard about "the only two genders: male and female" growing up, and now they think there are only two genders.

Gender non-binary is a very new concept in our mainstream culture; we all have to go through a lot of assimilation and accommodation just to start understanding; gender isn't about you genitals and there's more than 2. Everyone has their own idea of gender and of where they fit (or where they don't fit). When I try to explain gender to my friends I explain it like a spectrum (like sexuality) just as a wide statement. For most people I'll use their preferred pronouns (it costs nothing to change a pronoun to make someone more comfortable in a space) and their gender, how they came to their conclusion of their identity, isn't really any of my business or interest- but if the situation is right you could always ask a none cis person- learn to be a better ally.

because people mainly know about the binary system of gender and aren't getting the education they need to move from that to a system for all genders.