So many times, when people challenge the idea that trans women are women - they bring up that trans women were born male and have XY chromosomes. They are correct, but that still doesn't invalidate trans women.
In this article I will be S-P-E-L-L-I-N-G I-T O-U-T for you...
Biological Sex
Biological sex is (on the most part) binary.
You were either born with XX chromosomes - making you female.
Or born with XY chromosomes - making you male.
Now, we cannot argue with science... and the transgender is not trying to argue with you about the principles of sex and reproduction. We sat through the same biology classes as you - we get it.
Trans women are not female.
Trans men are not male.
Gender
Gender is different from biological sex.
Gender is not predetermined by any factors. Any individual, irrespective of their biological sex, hormone levels, or physical characteristics can identify as any gender (or the lack of gender).
A female individual identifying as a woman - makes her a cisgender woman.
A female individual identifying as a man - make him a transgender man.
A male individual identify as a man - makes him a cisgender man.
A male individual identifying as a woman - makes her a transgender woman.
Note how the person's pronouns are based on their gender identity - not their biological sex.
Acknowledging that biological sex makes trans people who they are
If transgender people did not acknowledge their biological sex - they wouldn't be trans. They would simply be women or men.
The fact that transgender women are male - makes them transgender.
The fact that transgender men are female - makes them transgender.
Trans women are women | Trans men are men
The word "woman" refers to how the individual identifies as.
It is true, that most female individuals identify as women, but that doesn't mean that a male individual cannot also identify as a woman.
It is also equally as true that the majority of male individuals identify as men, however that doesn't mean that a female individual cannot identify as a man as well.
The word "woman" encompasses both cisgender women and trans women. That's why we say that trans women, are women.
The word "man" encompasses both cisgender men and trans men. That's why we say that trans men, are men.