The way you present yourself can add to gender. If you prefer stereotypically masculine clothing but you have a female body, it might be harder to buy said clothing.
Also, I’m sorry that trans people “make it an issue”, they’re just trying to work on the fact that they can’t look at themselves sometimes and see someone who’s actually them.
And yeah, gender isn’t fluid- not for everyone anyways. A majority of people are cisgender and identify as their birth gender, but there is the trans, specifically genderfluid, minority of people, that gender is fluid for.