Gender fluidity exists given that gender is not a purely biological concept; there are lots of social/psychological aspects to gender, and if someone identifies as neither male nor female the "third gender" concept of sociology kicks in, although you do admittedly get annoying people who claim to be gender fluid as a means of being an attention seeking and/or to use as a weapon against their parents (I have someone who used to be a friend IRL who is like this, and honestly it saddens me 'cause it completely undermines all the work which is being done to try and secure the equal LGBTQ rights that he monopolises every conversation he's in over whilst making him come across as a complete hypocrite).
That said, the idea of gender being described
purely as a social construct as opposed to partially is something which I have always been a bit baffled by personally; while I don't disagree that there are definitely social aspects to gender as a construct, I fundamentally disagree with the idea that there aren't biological reasons why gender roles exist. I don't think that the concept of gender is simple enough to be described as either solely a social construct or solely a biological construct, and it's why I have never really liked people using the defense of "gender is a social construct" when discussing/debating topics surrounding it: it is a statement which tries to put a three-dimensional concept onto a two-dimensional plane with the intent of winning an argument, artificially creating a social high ground, getting a point across, or whatever other motivations people have for chucking the statement out without really thinking too much about it.
I believe that, as a construct, gender is an independent entity which is loosely tied into sex (a purely biological construct) in that there are minor differences between biological males and biological females who grow in a typical pregnancy (keyword: typical), ranging from the presence (and potentially ratio) of estrogen/testosterone to simple genetic differences, which alter behaviour in a typical member of each sex group (once again, the keyword here is "typical") both for the sake of maximising the success of the "sexual cycle" (there's probably an actual term for this which I cba to look up or remember) and for other reasons which likely tie into survival mechanisms we have used in the past. To take the simplest example of gender stereotyping (pink being girly, blue being boyish), there are studies which suggest that girls may be hard-wired in such a way which means they are more attracted to pinker shades than boys are (for example, take a study from 2007, for which the findings are explained in a reasonable level of detail
here or in a much more condensed form
here), and one of the theories behind its purpose is that our female ancestors did more berry hunting and, consequently, needed a way of having their eyes drawn to the ripe, red berries as quickly as possible, with people and primates generally noticing things that they like faster than things they don't like or are neutral about; another theory (this time my own) is that it may tie into the big, pink bums that baboons use to attract females, with the pink-loving mental wiring being shared. There is definitely the aspect of marketing behind the the gender association of these colours, for sure, but it is likely that these very marketing companies did similar (albeit less detailed) types of research before turning it into the mainstream cultural concept that it is today.
You can also see from research into gender dysphoria/gender identity disorder (GID)--a disorder which affects a person's sense of both gender identity and sex identity--that there is a link between the exposure to testosterone/estrogen in the womb and being transgender, with data collected from different types of twins indicating such a link--in a small study of around 51 pairs of twins (23 monozygotic (from one zygote) twins, 21 same-sex twins from different zygotes, 7 opposite-sex twins), all of the opposite sex twins showed signs of GID whereas nine of the monozygotic twins showed signs of GID (
source). Obviously not every opposite sex twin is going to be GID--I'm an opposite-sex twin and neither me nor my sister are GID--but the fact that every one of the ones who were looked at in the investigation were concordant with GID is unlikely to be a coincidence despite the admittedly-small sample size. Being transgender, in turn, means that a lot of a person's characteristics are very typical of someone of the opposite gender, and while not all cases will relate as heavily to hormones as others, the investigation into twins which was done here does suggest that being in the womb with someone of the opposite gender can lead to exposure to unusual amounts of the other hormone and, consequently, may have a biological effect on gender identity.
While there is a large amount of biology behind it, ultimately you could be exposed to a large amount of estrogen in the womb and still feel male, which is why there are large social aspects of this; not everyone is born in such a way that means they will identify as one gender or another, and while biology is likely to have some kind of influence on someone's gender identity, I think a lot of people have to go through the process of learning about themselves and ultimately working out how they identify in society. I think I'm kind of girly in a lot of aspects, but I wouldn't ever identify as anything other than male, and I don't think I needed as much consideration to reach this conclusion as some people do. Some people take longer than others, and the concept of gender fluidity only ever comes in after you have gone through this process of personal discovery, and it makes sense that there are people simply don't feel male or female. That's totally ok too! Having a "neutral party" (for lack of a better term; I am aware that "neutral party" is a pretty terrible way of putting it) is important for ensuring the success of a lot of LGBTQ rights movements as well as when discussing because it provides a view that honestly is pretty hard to imagine for a lot of people. That said, I do think that a lot of people--especially when they're younger and less sure of their identity, sexuality etc.--do confuse having some aspects which are typical of the other gender with being genderfluid, and this is honestly something that a lot of people who argue that it doesn't exist like to pick up on and milk just because it is a low-hanging target for criticism of the concept. It comes back around to the thing I mentioned just a few lines up about how you can be a girly boy or a tomboy without actually identifying as the opposite gender or even as a third gender.
This post doesn't really have much of a goal to it; I started typing something and kind of extrapolated from there hahaha. Matters like this are important to talk about, so I just wanted to state my thoughts on it.