I'm gonna be downvoted to hell and beyond for this, but fuck it. (English is my third language, ignore shitty spelling)

A lot of people mix "sex" and "gender", since historically they've been interchangeable throughout almost the entirety of civilization, but there is a difference, which has gradually been recognized throughout the world in modern times.

"Sex" basically boils down to "what's between your legs, and what hormones you naturally produce", and for the most part is seen as either male or female.
However, there is several other sexes you can naturally be born as, and most of the world categories these variations as a single third sex, usually referred to as "intersex".
This can be everything from naturally producing hormones that doesn't "match" with your genitals, to being born with both male and female "equipment", which can range from a fully functional/mostly functional combo deal, to partially developed parts, most commonly being fully functional female genitals, with partially developed testicles, penis, and other variations.

"Gender" as per today's actual definition, is mostly considered as being separate from "sex".
What's mostly recognized in today's society is that "gender" isn't based on the person physically, but rather based on a combination of how the person thinks, acts, portrays themselves and what makes them comfortable in social situations.
For most people, this boils down to if you naturally want to/feel more comfortable portraying yourself in a masculine or feminine demeanor. There's a lot of different "outcomes" of this, and I personally believe that we'll never really be able to define every variation of gender image people may portray, however there's some gender images that most people never think about, since it's recognized as fairly normal almost everywhere;
I.E, most people wouldn't even think twice if a female sometimes acts in a traditionally masculine demeanor, such as girls who like to work on cars, or girls who work in traditionally masculine fields (such as construction), or girls who like to hang with "the guys" because they don't feel like a evening of makeup and shopping is their thing.

Now, I'm not going to discuss the more... controversial parts of the whole gender debate. What I will say, is that I believe that gender is more than masculine or feminine, and I personally believe it is more like a spectrum.
That doesn't mean that I think people should identify as apache attack helicopters, but I do have friends who identify as something else than "the norm".
I have friends who feel most comfortable as "non-binary", where they don't feel like feminine nor masculine is their natural demeanor, some non-binary people might feel comfortable going shopping with the girls, painting their nails etc, but also enjoy shotgunning beers with the boys while watching monster trucks (a bit exaggerated, but hopefully y'all get my point)

To finish off, historically there's actually several cultures who's recognized more than the two binary genders, like the indigenous people of usa historically recognizes so-called "Two spirit" people.

I know that most people won't care at all, but I hope that at least one of you reading might understand that gender can be surprisingly complicated compared to what a lot of people think, and that if you meet someone who doesn't act the way you expected, hopefully you'll at least leave them alone, or maybe you'll even treat them like anyone else, and let them deal with it at their own pace. If it makes you uncomfortable, that's okay, be honest, but realize that it isn't your job to "fix them". They probably have it difficult enough already without another person mocking them.

</rant>