Because so far I understood that for example the sex can be female, and so can the gender. But what sense does it make to have female gender? There is just a human in your head.

But then again… how important is this belief? And why are people claiming that there are more than two genders? Because of gender refers to sex, then what is going on there?

Their are neurological and therefore psychological differences between people of different genders. These are normally aligned with their physical sex and hormones. When there is a misalignment between gender and sex that’s what trans people are. It’s not belief, it’s brain structure.

There is just a human in your head.

Do you not feel attached to your birth gender at all? Or any other gender? Like if you woke up in a body of the opposite sex would you be okay with that?

Statements like this sometimes make me thing the person I am talking to is either very ignorant or a very confused agender person.