well you’ve sorta answered your own question here

if you aren’t really sure what gender is, it’s hard to identify definitively as one or the other

how can someone identify as “male who does unmanly things” when the only way you thought to even express the concept was with “unmanly”, i.e. literally “not like what males do”?

if sex and gender are different, then gender must be a description of how we think and feel and act.  there are only two common names of genders, so the only possible conclusion is that all of human thought and emotion and behavior is being sliced into one of two mutually-exclusive boxes.

queer/fluid are attempts to exist alongside this clunky system by people who have a hard time ignoring it or fitting into it