Enough of all this "gender-fluidity" bullshit - you want to identify as a man, a woman, a transgender, or a tree then go ahead and identify as whatever you want to identify as. Nobody should care that much, and we shouldn't try to put a label on everyone. Genders have become a symptom of special snowflake syndrome. Just get rid of gender entirely and call yourself a human being who has a certain genitalia. — darthbarracuda

So the social construction of gender is based on the idea of binary. There are men and women. Men have a penis and women have vagina. However, as we know it is definitely more difficult than that. There are more than enough people who simply don't identify themselves with their biological gender.
Another problem is that you don't just simply accept the biological gender for what it is, you interpret it. Over the centuries certain images of the genders have therefore been established. Women according to this have certain characteristics whereas men have others.
If it weren't for the gender binary there wouldn't be transgender people. Same goes for all sort of gender identities that go beyond male and female. Gender fluid people in particular either don't feel like any of those characteristics are bound to their gender identity or one day their identity resembles the characteristics of a certain gender more than the other.
Saying that gender fluidity is nothing more than a person trying to be cool, or trying to be a special snowflake is simply obnoxious. Because it is denying the problems that come with being gender fluid or any gender identity other than male and female. It can be difficult and can lead to a existential crisis, mental health issues, being bullied, ...
The problem is that you can not get rid of gender but you can try to understand that there is more to the gender spectrum than male and female and accept a person's identity even if it is seemingly very obvious that someone is just trying to be cool.

And why would you want to stop thinking about gender? It is certainly a very important topic of our time since it reflects the human race with its history, science, philosophy and language