You don't need sources, dear.

How are babies made? A sperm meets an ovum. There is no third category, no "Spovum". It's absolutely binary.

Where do sperm and ovum come from? From testes and ovaries, respectively. No ovary can produce a sperm, no teste can produce an ovum. Never ever ever ever. It's absolutely binary.

No human has both functional testes and functional ovaries. No human has ever produced both sperm and ovum. No human has ever switched from one to the other. It's binary.

The act of producing new people is super, super important. All other human achievement depends on it. So the two, absolutely distinct groups - sperm producers and egg producers - are really important categories. The former are called 'men', the latter are called 'women', and the two groups have distinct body shapes, genetics, physiology and hormones.

There are also some people who produce neither sperm nor ovum, but they usually HAVE either testes or ovaries along with the standard body plan for that sex, so it's really really obvious which group they belong to.

Up until this point it's all basic 'birds and bees' facts. You don't need a scientific source to tell you, and anyone who acts like you do is being willfully stupid.

There are a handful of people who have a condition that results in having neither testes nor ovaries (complete gonadal dysgenesis) and you could truly say they're neither sex but they have the same overall body plan as women so we group them with women.

There are an even rarer group of people who belong to one sex but appear to belong to the other (complete androgen insensitivity syndrome). This is the group for whom the term "assigned female at birth" was intended for. With a very simple investigation doctors can tell that they are in fact male. Although they externally appear female they have testes and the interior body plan of a male. We may treat them as if they were female but we're 100% clear that they are actually not.

Sex is absolutely binary. It's the two categories involved in making a new person. Gender is the set of rules, expectations, and imitations we put on the sexes. Gender is arbitrary and changes over time and across cultures. Gender identity is a spiritual belief that a person can be born with a 'girly soul' or a 'manly soul' and when they are born with the 'wrong' one they can tell and they often 'need' surgery and hormones to make their body 'match' their soul.