Here's PM_ME_YOUR_TITHES' top answer in a recent similar thread on /r/changemyview (actually I recommend you just go read the whole thread, it's a wealth of information and information correcting):

Well, to start off with, there's a big divide between biological sex and gender. I highly recommend you research this more, but I'll try to touch on the basics.

Sex is rooted in an organism's DNA, and is defined by which reproductive cells (sperm or eggs) they produce. Animals with testes who produce sperm are biologically male, and animals with ovaries who produce eggs are considered female. You're probably familiar with X and Y chromosomes, which is how sex is determined in humans, right? Actually, things go wrong. [Swyer Syndrome]() is a big example, in which a person has an XY chromosome but something goes wrong with the Y. Humans are actually female by default, and the Y chromosome is required to activate in order to form male parts. Without it, people with Swyer develop uteri, vaginae, and other normal female organs. However, they have streak gonads which don't produce sperm OR eggs. So what do you call them? Their chromosomes say male. Their organs say female. But they can't reproduce as either one. This is just one of many examples of conditions that show how sex is not even nearly as straightforward as it seems when everything goes normally. So, this is only half the problem. Now we need to address gender. While there are several definitions of gender, some of which address biology, the accepted definition in most of the social sciences is the combination of external and internal social features that are associated with masculinity or femininity. These usually, but not often, correlate with sex. But for obvious reasons, people who lack an easily definable sex might also be hard to correctly gender. But, clearly, these people aren't genderless, they still have elements of masculinity and/or femininity. That leads us to the question: if gender can be rooted in something that isn't biology, isn't it always rooted in something else, at least partially?

The easy answer is a person's feelings. Hormones, body structures, socialization, and a thousand other things can feed into how we perceive our gender, leading to the inevitable conclusion that gender is determined primarily by how we act and how others act towards us. Imagine, if you will, that you find out tomorrow you actually have two X chromosomes. I'm assuming you're male, or at least you think you are. But for all you know, your genitals are the product of elaborate surgery when you were an infant- you actually were born with a vagina and streak gonads, but your clitoris was large enough that doctors were able to turn it into a penis and give you testicular implants. I'll admit, it's far-fetched, but in the end it's very hard to know our sex for certain until we've conceived a child or had serious testing done. But let's assume it happens. Would you immediately ask for breast implants, get your facial hair permanently removed, and start dating men while considering yourself straight? Probably not, right? Your gender goes deeper than just what you understand your body to be. Similarly, if you were abducted by aliens and had your sex switched against your will, do you think you would still identify with your prior gender?

Now, don't get me wrong. There's a heavy correlation between sex and gender for a reason, and part of that is biology. Testosterone does correlate with aggressive, masculine behavior. But there are plenty of aggressive women, right? And while pregnancy does cause mothers to form a tight chemical bond with their children, there are fathers out there who form just as close of an emotional connection without being pregnant. Our bodies might make it easier to act as our gender, might give us the right chemical cues and biological urges, but they do not define everything we feel.

Furthermore, gender isn't even so much about feelings as it is the way we act. Crossdressing proves this- a convincing crossdresser knows how to present themselves as the opposite gender, to the point where few people will ever guess their sex doesn't "match up". Have you ever wanted to wear a dress, just to see what it's like? Wished someone would bring you flowers or pick you up for prom or be the big spoon? Maybe you wanted to wear pink or high heels or makeup? You could even enjoy baking or childcare or color-organizing your sock drawer, for all I know. Ovaries don't make someone any more likely to enjoy the color pink than testicles do, these things are based in our personal preferences and how we were socialized as kids.

Being transgender means that your genitals don't match up with the gender that you relate most to. Most of us have some preferences and behaviors that don't line up with our gender, like tbulldykes. They are clearly able to have these masculine feelings, preferences, and behaviors while still identifying as a woman. However, is it so hard to believe that they could have a few more of those preferences, pushing them into predominantly masculine territory? People fall all over the spectrum; we do a lot of policing to tell children what they're "supposed" to do and like, but if you watch toddlers at play when they're still too young to have this solidly internalized, you'll see there are a good number of girls who run with the boys and boys who play house with the girls. Being transgender later in life is just like that on a more complex scale, having preferences that are mostly feminine when you have testes or masculine when you have ovaries. Basically, our bodies don't define what we think, feel, and do. From a biological standpoint of sex, maybe we really can't change. But from a social and cultural point, it's not only possible, it's fairly easy.