I’m working from the understanding of physical sex as the bio-bits

In a purely physical perspective, sexual characteristics don't always fit in a neat binary though, and they can also change.

It's not that simple though, because there's a whole social structure attached to it. The social structure insists that sex is binary, and enforces roles and rules based on perceived sex. Another part of the social structure is the importance placed on sex. Left and right handedness is also a physical characteristic, but it's not something you use to categorise people in your mental rolodex. If I ask you about your friend Alex, without thinking about it, you'll be able to tell me Alex's sex, because it's something you are taught matters, but it's a flip of a coin as to whether you can tell me whether Alex is left or right handed. And that reason for that is all down to the social importance placed on sex.

So yeah, sex is "bio bits" but probably not in way you're thinking, and it comes with a whole bunch of social stuff too.

If we take gender as being an expression of your persona

It's not.

then what function does a title/pronoun serve?

The pronouns people use to talk about you, are indicators of the social aspects I was talking about before, and a direct line in to how people perceive and "categorise" you.

We’ve had men or women who enjoy things traditionally associated with the other gender for as long as there have been people I expect. If that’s the case then what purpose does the need for a gender title serve?

I'm a trans woman. I don't particularly enjoy things associated with women. I'm don't understand femininity, and most of my interests are masculine coded.

Which is to say, this stuff has nothing to do with my gender.

It does relate to the social expectations of sex and gender, which means that they're important to many folk, but they aren't gender.

I’ll admit personally questioning some things like fairness in cis/trans integrated sports

Don't. The whole conversation is driven by transphobes trying to use overly simplistic and misleading representations to normalise the exclusion of trans folk as a wedge tactic, before they move on to exclusion in other areas. If you don't know much about it, it's impossible for you to have an informed opinion on the subject, and that can lead to a lot of very real harm and exclusion to trans folk.

A few thoughts on subjects that haven't been touched on a ton or framing which might help you understand some of the points you've brought up:

• I think it's important to note up at the top that all words are made up and definitions are merely attempts at society to agree on what a word means so that we can communicate with each other. The presence of slang, the creation of new words, and the shift of the definitions of words over time are all important factors when we talk about the deep specifics of a particular topic or idea.
• Nearly everything in this thread is about a topic which broadly falls into the category of "loosely defined social concepts" more formally known as social constructs. Examples of loosely defined social concepts include: gender, romance, beauty, family, race, wealth, trendiness, class, art, and status.
• Social constructs exist on a spectrum, with some having stricter definitions. For example, dictionaries exist in languages because additional structure is useful. Currency is often defined by governments to help more directly understand wealth or money so that individuals can exchange on equal terms and so that individuals can be taxed.
• Sex and gender used to be interchangeable words in western society, back before we understood any "modern" science which delineated the two.
• Over time sex became a legal and medical term, to describe people who were assigned female at birth generally by genital inspection of the doctor or whatever was recorded on the birth certificate
• Gender theory, or at least the modern roots of it, emerged during women's suffrage in the united states as a way to separate the social factors from the biological ones - to provide framing to examine social pressures, social norms, social ideas as a construct and not innately biological
• Modern gender theory importantly separates gender identity from gender expression. Much of the discussion in this thread about gender nonconforming individuals such as tomboys being different from trans masc individuals comes down to this framing and their assigned sex at birth. Strictly speaking, having a gender identity which does not match the assigned sex at birth can be considered trans. I say "can be" because labels should never be forced on someone else
• Labels are personal, and therefore messy, and do not always neatly match with definitions for words that are in dictionaries or generally accepted in whatever social circles. For example, a person who has a gender identity of non-binary, who presents very feminine, could still identify as a transmasc individual as an explicit recognition of their internal sense of gender or the steps of transitioning they may have taken.
• Titles and pronouns and honorifics are individual preference and are not strictly gendered. Take, for example, the historical use of words such as lord, king, grace, duke, doctor, baron, viscount, jester, chief, lieutenant, esquire, the honorable, elder, sensei, the wise, acolyte, apprentice, etc. - these are used to signify a specific role in society or someone's personal preference. Unsurprisingly, people can often have feelings about the use of these words
• If you or someone you know happens to have a nickname or another name they go by in certain contexts or overall, it might help to reflect upon these names and the reason they are used. In some cases, they are forced upon people and undesirable, such as nicknames that come from hazing or bullying. In other cases they are adopted for any number of reasons, including that the person just doesn't like their name or prefers this one. Think about how the person who uses or has these names used on them feels about their usage - this same framing can be used when it comes to pronouns or just general perception by others in a society.
• A lot of the framing in this thread is on the gender binary, or genders created out of the sex binary (importantly, not a true binary in any science... nature is messy). Attempts to understand non-binary individuals through a binary lens will necessarily fall flat as these individuals do not see themselves as existing within the binary.
• Gender identities which are non-binary are often based on one's gender identity - which is also a loosely defined word. A sense of self ultimately likely comes from feelings, and just like some people feel strongly that being a mechanic is a masculine trait, people might feel that literally anything is gendered and their gender identity is composed of those feelings. Thus even things which binary folks don't generally consider to be gendered may be an important part of one's sense of their non-binaryness.

You know this meme?

This is essentially true because (we) trans people have to spend a ton of time into thinking about what gender and sex are. What may be helpful is thinking not about how trans or even queer people deviate from the norm but how sex, gender and sexuality work in general.

You may want to look into Judith Butler's revelatory "Gender Trouble" (or at least summaries of it) that was kind of the birth of Queer Studies and where they discuss how gender/sex are "performative" (not in the sense of literally performing but rather how it is repeated and maintained). Butler explains how the category of sex is not descriptive but instead a constructed one. Even a newly born baby is put into a constructed (made-up) category that doesn't necessarily reflect physical reality.

Sex as a physical reality actually crumbles the moment you have a closer look at it. Medicine has been trying to correct it by mutilating intersex people for ages now, trying to put them into neat categories. Like someone already said here in the comments, you probably don't know your karyotype, right? You just assume it. There are various factors playing into sex, like chromosomal, hormonal, genital sex (plus some more!). All of them can show variations.

How sex is constructed can be made even more clearly when looking at animals. Biologists have always been very eager to put animals into sexual binaries because they tried to replicate their own view of the 'natural' man vs woman binary. But this is far from true. There are some animals with two genders, but also some with three or more and there are some with only one. Intersex, "trans" and queer animals are very common among animals as well. E.g. there are female deer with antlers etc. Science has just been too busy with projecting their own 'truth' to realize this. In recent years we have been catching up though. There is this great book called "Evolution's Rainbow" where the author Joan Roughgarden goes into much more detail.

Another interesting point is that while it feels "natural" to us that there is a gender binary, this is actually a pretty modern view. Gender and sex as we know it have only been around for a few hundred years. Just like homosexuality and in response heterosexuality are very recent phenomena. Likewise, our concept of "love" is also a very recent invention and probably a product of the emergence of capitalism.

But you can see how the performative nature of sex and gender leads to scientists and generally people trying to impose/project sex and gender onto very arbitrary traits or phenomena. And by doing so, the perceive differences between sexes/genders is even stronger which leads to repetition of the imposition/projection. We probably cannot know how our species would express itself without these social constructs interfering.

So, that is why some people here in the comments gave you the wise hint not to try to rationalize gender/sex. Those are social constructs all the way and won't really ever make sense. Where does the feeling to be any gender come from? No idea! But it definitely is there (or not haha).

Lets see if I can explain this clearly enough on the first try.

So your question is, "If we take gender as being an expression of your persona, a set of traits that define one as male, female, or some combination of both then what function does a title/pronoun serve?"

Well, this is both weirdly complicated and absurdly simple. Gender isn't just a set of personal traits, but also a social concept. In that framing, titles and pronouns are a signal that one gets from other people in society that tells them how their visible presentation is being perceived and interpreted.

One can relate this to the social distinction between the nobles and commoners of yore. Even though the only visible differences between the two groups are their attire, for a person that sees themselves as a member of the aristocracy to be spoken to as if they were a commoner can be a grave insult.

Likewise, most people find it discomforting when their innate sense of their own gender is contradicted by the people around them. Specifying our pronouns/titles when we introduce ourselves can provide context for folks who might have otherwise assumed someone's gender incorrectly.

I'm non-binary, meaning I am neither a woman nor a man. I find it deeply uncomfortable when people call me a man because it assigns a bunch of expectations onto me that I know I don't fulfill and that I don't want to be associated with.

To assume that some things are masculine or feminine traits seems to put unneeded rigidity to things.

I completely agree, which is why I have broken free of the shackles of gender. (being hyperbolic here, but it's kinda how I feel). If the world were perfect, everyone could do what they want and you could meet a new person on their terms instead of assigning a gender and expected behaviour onto them based on their length of hair and style of clothing. But, we live in the world we live in, and so there are certain things that are deemed to be masculine or feminine.

EDIT: I've been thinking of how to explain this to someone who doesn't feel the same way I do. Hopefully you are a car driver, because it's the best analogy I can come up with. Imagine you're watching dashcam footage and the car you're riding in is speeding when a young child runs out from a parked car. For me, my right leg would immediately jerk to try and hit the (non-existant) brake pedal. My gut tenses. I grunt mentally and can see everything ending in disaster. All of this happens instantly before I can process or think and realise I'm just watching a recording. This is kinda how it feels when someone calls me a man. It's just a visceral immediate dislike and feeling of deep uncomfort in a very similar way

To me it seems like the important question is:

Why wouldn’t one do something that makes others feel valid/happy/comfortable for so little effort?

It’s easy to respect name and pronoun preferences and admit when mistakes are made. One needn’t to dive into the full nuance and complexity of trans experience to understand that.

rather than talk about my experience, i tend to think it's more helpful to have cisgender people imagine themselves in similar hypothetical situations:

1. imagine everyone in your life started using she/her pronouns for you. how would this make you feel? how does this affect how you react to them? how does this affect how they react to you?

2. imagine you woke up in a female body. how would you feel? imagine you had to adjust to it for a week. then a month. then a year. then ten years. what adjustments to your life would you have to make? how does this affect how people treat you? how does this affect your behavior? how would you feel about this situation? what would you miss about your previous body?

the second experience is essentially what it's like to be a transgender man, except it's from birth and you don't (necessarily) get to start with the knowledge you're a guy (some people more intuitively figure it out than others)

these questions hopefully help you develop a sense of what gender means to you, which should help you understand what gender means to us (obv it's not the same for everyone, and it's pretty binary, but it's a decent starting point)

My favorite part about the Dunning-Kruger is that so many people get it wrong.

This graph isn't even close.

I'm sure other people in this thread have done a great job talking about the mainstream stuff, so I'm gonna get weird.

Nonbinary identities aren't all combinations of male and female gender traits. There are also nonbinary genders. They're called xenogenders. And nonbinary identities can be any combination of male, and/or female, and/or any number of xenogenders. Xenogenders have traits that you may not typically believe are gendered, or that take the stuff you know in unexpected directions. But what are you expecting from a gender that isn't like anything you know? It's gonna seem weird.

Xenogenders are common among otherkin and alterhumans. Otherkin are people who don't identify as human. They may identify as a real animal, or a mythical or fictional creature, which may be sapient or nonsapient, though nearly all otherkin are themselves sapient. Alterhuman is a broader category that also includes people who only identify partially as human or nonhuman. Perhaps someone who remembers a past life as a wolf, for example.

Sometimes a brain contains more than one person. The brain contains a mechanism for creating a person, an identity, a consciousness, and usually it uses that mechanism once. Sometimes it's used more than once. It's called plurality. Plurality can be the result of traumatic mental disorder, but it can also be healthy.

Among other social constructs such as gender, as useful as they can perhaps be when looking for a generalisation of "what are the terms for you to be understood in?" I have recently been questioning sex. Sex is often referred to as the biological bits, but is that true? No, because it's an incomplete picture.

Biologists seem to currently accept sex is a mosaic of sexual characteristics. This includes but is not limited to genitalia and chromosome—the two most thought about elements I'd wager—and your chest, your hormone balance, but also measurements like around the hips, waist, shoulders... And of course, your role in reproduction, especially if you can reproduce.

Many of these characteristics are mutable, especially in today's society with hormones and surgeries. Functionally speaking, they don't matter, we as a species are not at risk of extinction and simply do not need to care about it. Sex was fraught even as a measure of reproductive capabilities anyway. We should care for each other's happiness first and foremost.

But even mutability aside, sex isn't consistent between men and women, with different hormone balances and even some variations in chromosomes or the capacity for sexual reproduction. Also, see the existence of intersex people, who, by their existence alone, shatter the binary.

I don't believe sex is a useful categorisation. Sex and gender and expression and the things you enjoy are different, but they're also both still constructions with your presumed gender being extrapolated from the most visible elements of your sex and huge variability for each person therein, but the correlation is starting to feel weak.

Sex and genders, as structures, are the product of cisheteropatriarchy, ie sexism. Even in sports. Social constructs generally arise as a necessary division for societies to make given their material conditions, and it was used to increase populations. I would say it is time to leave such vestigial logic behind.