Chatgpt is wrong here, sex is more like a series of bimodal bell curves measuring traits like gonad type, chromosomes, hormone levels, secondary sex characteristics, neurobiology, and probably some more I'm forgetting. For each trait, one bimodal peak can be labeled something like "typically male" and the other "typically female". For instance, hormones would have "higher testosterone" for one peak and "higher estrogen" for the other. You can usually categorize male vs female by weighing where an animal falls on these bell curves across all traits, but that's more of an art than a science, since the scientific perspective is more "sex is a composite profile" than "sex is a binary to be categorized".

That's why you always hear people say 'it's a spectrum" or "it's socially constructed", because that's the easiest way to explain it in simple terms (even if it is non-descriptive and annoying to hear as a shibboleth)

Dingus, you mixed biological and social gender. Biology usually talks about biological genders.

Not sure if you're joking or being sarcastic, but here are a few examples where the mere absence / presence of a Y can't determine sex & gender.

• Sometimes a person has XY chromosomes, but the body developed in a female manner because the fetal cells were resistant to testosterone. Such a person has good claim to being a woman (she developed that way) or a man (he has a Y chromosome; his brain could have been sensitive still to the testosterone to still develop male-coded)
• Sometimes a person has XX chromosomes, but the body developed in a male manner. Usually (though not always) this is caused by part of a Y chromosome ending up on an X. Such a person has good claim to being a man (he developed that way) or a woman (maybe she lacked enough testosterone to male-encode the brain).
• Other conditions such as XXY combinations, or chimeras (some cells XX, some cells XY), or other intersex conditions where some body parts develop male, some female.

To me personally, I view trans people as a type of intersex person. It seems entirely possible that you might have a person whose brain cells were more or less resistant to testosterone and/or exposed to testosterone and truly is a man/woman in a woman's/man's body. You don't need to bring choice or culture into it - I think biology alone provides good evidence to believe trans people about what gender they are.