We are pattern-finders. I first encountered this fact in the form of ritual and dogma while studing comparative religion and social anthropology at university. It was a simple insight really, very self-evident when you think about it, but it transformed the way I looked at the world. The book that started it all for me was Mary Douglas' book Purity and Danger, old by now but I think it still holds up well.
But this is old hat: humans are really, really good at finding patterns in nature. It's vital for survival to be able to quickly sort things into categories: that's an animal you can cuddle, that one will probably eat you. Those berries are edible, those are poisonous. That person is a member of the flock, that one doesn't fit in and might hurt you. Very handy. It's also the function that creates the fear of the Other, the impulse to shun or attack anyone or anything that doesn't belong to any of your approved categories; it's the function that helpfully tells you that this person belongs to group X and therefore has Y attributes. Or: this group is good, the other is not-good. The other group, the not-good, is scary. Even more scary are things that will not easily fit into those categories.
Again, old hat to Swedes but a good example: recently a gender-neutral pronoun, hen, popped up in the Swedish vocabulary. Since Swedish doesn't have a pronoun for situations where a person's sex is irrelevant, outside of the binary category, or just unknown, this was a very welcome addition. So far so good, yes? But a lot of people went apeshit over this. The two most common arguments were the following: hen is a word for poultry in English (somehow the people who used this as an argument had no problems with other homonyms like fart (speed), slut (end) or barn (child). The other popular argument was that this was all a plot by the feminist mafia to erase gender. Other super scary occurrences are people who don't fit into the binary gender categories or move between them; people who don't fit into the stereotypical hetero- or homosexual categories; people who can't be identified as belonging to a particular "race"; people who don't quite resemble our image of a "standard human" (I once knew someone who had a phobia of dwarves, for this very reason). It seems difficult to accept that reality is a spectrum rather than a grid.
But this isn't specifically about gender and sexuality, it's just the first example I thought of. Other examples: the uncanny valley, or the unsettling area where a doll looks like almost human but not quite. Clowns fit in here too. They look human, but there's also something else to it: they wear a mask, and you can't be sure there's a real person under there. They might be something else, masquerading as humans. I was terrified of Santa as a kid. In Swedish tradition, Christmas presents are handed out by an older family member (usually the father or grandfather) who's dressed up as Santa. The first time I was old enough to understand what was going on, my grandfather showed up in a Santa mask, and I was terrified. I wouldn't stop screaming until he took the mask off. Because he looked human, but I wasn't quite sure that he was. I wasn't sure those were his eyes behind the mask. (but of course there's more to masks. Note to self: write about masks, possession and mask play sometime. That also ties in here.)
The food rules in Leviticus? An amazing exercise in sorting animals into categories and shunning those that don't fit in. If an animal has cloven hooves and chews the cud, it fits into one category; if it chews the cut but doesn't have cloven hooves, or if it has cloven hooves but doesn't chew the cud, it falls outside of the category and is shunned. A water-living creature is okay to eat if it has fins and scales, but not if it has scales and not fins, or fins and not scales.
So where am I going with this? I love the in-between categories. It unsettles the hell out of us, and at the same time we're obsessed with it. I know I am. Much of what I write is about that which falls between the boxes. People who look human but aren't quite so; voices on the telephone that sound human but you can't be sure; objects that look like objects but will cease to be so unless you tell them what they are. And amazingly, despite how much I write about it, it still unsettles me. I suppose because my brain, and your brain, is wired this way. A useful but scary-as-hell function when not aware of it, but a playground when you are.
I'm still afraid of Santa masks.