How are there more than two genders?

Uh. Ok. I am going to quietly exit this conversation. Good luck with your revolution.

Multiple problems with this - firstly sex isn't exactly a biological binary, or even trinary. The traits exist and then sex is a taxonomical model to try and group those traits (and like all taxonomical models it has it's own flaws). So, science doesn't say there's more than two sexes but it also doesn't really state there are two sexes as such because sex is also a cultural concept (the expression of which is gender).
Now, the cutural concept of sex and gender is entirely inseparable from the empire of heterosexuality, and this is where the matfems turn up to spoil the party - man holds power, woman is constructed for his ends, as broodmare and bangmaid, with those who cannot or refuse to fulfil that role cast out of it (this is where the concept of lesbian as a gender comes from, read The Straight Mind by Wittig for a more detailed idea of the theory), now this also applys to trans people, particularly trans women who are not men, having stepped out of and been denied the power of manhood, but are also unable to fulfill woman as broodmare within the heterosexual empire and so become third-sexed.

Hetero empire? Men creating the gender binary cultural concept for their own gains?

Biologically we are born either men or women. Society can change norms about these two genders but why are the liberals normalizing the concept of more than two genders despite the fact that science does not provide any evidence of more than two genders?

For me, gender is a binary system. Intersex people are the only biologically non-binary people. The rest is what the liberal society enforces, not what people want to be. So why bother men and women with this notion? I'm just asking nicely.

Youre conflating gender with biological sex when the two are separate. Gender is culturally and socially defined (but can interrelate with biological sex). Its also not a modern or western concept. The Bugis people of South Sulawesi for example have had a concept of five genders for centuries if not longer (dating to the pre-Islamic period at least).

Yes. Without the existence of patriarchy woman wouldn't exist (and this is where I hammer the "Read Dworkin" button, specifically from Right Wing Women: "It is the imposition of the sex-class definition of women on women—by any means necessary—that devastates the human ca*pacities of women, making them men’s subordinates, making them “women.”") because womanhood is constructed as a category to be subordinate to men

If gender is a cultural concept. What is the scientific meaning of gender? Is gender in science binary? Because due to science (Biology) we are either born men or women.

Gender isn't a scientific concept, it's a social concept. This is widely accepted by biologists and anthropologists.
Moreover biological sex itself isn't a binary across living organisms, either in general or within a creatures lifespan. There are numerous examples of plants and fungi with biological sexes well in excess of two. There are numerous animals which demonstrate the ability to change biological sex in their life span (parrotfish for example). This is not contentious to biologists.
Even in the specific case if humans we know there are intersex individuals and this is not statically speaking rare relative to other common congenital conditions. And it is widely accepted by scientists and academics that sex and gender are separate concepts, and the latter is defined socially and culturally. You need only read basically any anthropology textbook or biological science textbook aimed at degree level students or beyond and its spelled out even on that level. It's not even a cutting edge research thing- it's just standard factual information in those fields.
Asking "what is the scientific meaning for gender" is like asking "what is the scientific meaning of marriage". It not only doesn't have an answer, the question doesn't make sense.

Biologically we are born either men or women. Society can change norms about these two genders but why are the liberals normalizing the concept of more than two genders despite the fact that science does not provide any evidence of more than two genders?
For me, gender is a binary system. Intersex people are the only biologically non-binary people. The rest is what the liberal society enforces, not what people want to be. So why bother men and women with this notion? I'm just asking nicely.

What about intersex people who are born neither male or female?

Gender isn't a scientific concept, it's a social concept. This is widely accepted by biologists and anthropologists.
Moreover biological sex itself isn't a binary across living organisms, either in general or within a creatures lifespan. There are numerous examples of plants and fungi with biological sexes well in excess of two. There are numerous animals which demonstrate the ability to change biological sex in their life span (parrotfish for example). This is not contentious to biologists.
Even in the specific case if humans we know there are intersex individuals and this is not statically speaking rare relative to other common congenital conditions. And it is widely accepted by scientists and academics that sex and gender are separate concepts, and the latter is defined socially and culturally. You need only read basically any anthropology textbook or biological science textbook aimed at degree level students or beyond and its spelled out even on that level. It's not even a cutting edge research thing- it's just standard factual information in those fields.
Asking "what is the scientific meaning for gender" is like asking "what is the scientific meaning of marriage". It not only doesn't have an answer, the question doesn't make sense.

PROSM Brilliant answer. But just cse you used too many big words
Sex is what you are, not just two possibilities
Gender is what you feel like - infinite possibilities
Identity is how you present yourself (image and / or dress) - infinite possibilities
Sexuality is who you fancy or not (way hey!!!)
And you can mix and match and at the end of the day it matters not a jot because on a planes or trains, all toilets are gender neutral and always have been! 😀
How dull would life be if we were all the same. I don't know... Like in North Korea for example?

The reason why I demanded scientific is because I would trust the science rather than political bias.

The reason why I demanded scientific is because I would trust the science rather than political bias.

"all toilets are gender neutral and always have been"
At least the toilets in our basic houses are gender neutral because toilets in public spaces like workplaces, schools, restaurants are usually segregated which isn't for cultural reasons but for safety reasons.

Safety? Because women couldn't just walk into a mans toilet or visa versa? Give me strength! Incidentals I went to a leisure centre today. Not just any leisure centre. It was a sub tropical paradise and surprise surprise - gender neutral changing rooms. And guess what? The women weren't being raped by the men! Even the children weren't startled!