Now... I am trans so this is my supposition and based on my discussions to cis people about the cis experience but I think there's somethings that we actually assume about cis people.

While some cis people seem to have a very solid gender identity that is in line with the tran experience of gender euphoria that seems actually kind of rare. I think a majority of cisness is actually defined by a lack of strong feelings about their body and their experience of gender. Their assumption of gender is simply the path of least resistance. Their experimentation when they have it is out of curiosity but it doesn't really resonate.

Do you think you would be upset at a foundational level if you woke up tomorrow in the body of the opposite sex and everyone had always known you as that way? Aside from maybe an uncomfortable adjustment of role and some interest in experiencing something different would you think that you would be terribly bothered? These sentiments can sometimes be construed as being a non-binary sort of thing - but the more I discuss the topic intimately with cis people to try and understand their experience the more that feels like the difference and particular advantage being cis represents. If you do not experience the pull and certainty of euphoria or dysphoria informing your cultural and biological needs that could be because what defines the majority of cis people is a fluidity and adaptability because of a near complete lack of preference rather than a preference that specifically matches a sex phenotype. Meanwhile transness can almost be defined as a surplus and rigidity of preference so we find ourselves trying to explain something like why we can't easily force ourselves to choke down food we detest to someone who maybe doesn't even experience taste.

A trans gender identity isn't exactly always subtly experienced. A correct gendering when you don't expect it might literally make your week. Disgust at your own body can make you run past the bathroom mirror so you don't have to look. It's not so much an "keep an open mind" kind of deal. You get a pack of stimuli like something entirely independent has a shock and reward system hardwired into your brain and you don't really control how you feel about it. When you talk about worrying about fluidity I think you are kind of concerned from the cis standpoint where being nebulous in gender is quite natural. From a trans community perspective we know we are on a journey where we are essentially learning the nature of the internal reward system. Our genders are reflections of that and we generally want to ditch whatever isn't working and update our coping mechanisms to account for changes or if we realize we didn't actually fully understand the exact nature of the trigger that was setting us off and can change our tactics accordingly to maximize functionality and happiness.

To further the parable a kid might after years of hating olives so much they spit them out whenever they eat them decide to try them again to see if their tastes have changed.. but it's not on parents to keep putting olives on the kids plate for them to "keep options open" when the kid can basically decide at any point to just reach across a the table and get their own olives if they want them. Maybe the kid knows they don't like olives because whenever they accidentally eat one it still tastes bad.

If someone keeps trying to hand you something you know you don't like and you keep having to passively or actively reject it... it can be really annoying and you generally stop wanting to be around that person.