A friend asked me "Should I treat you like a boy or a girl?"

As a cis guy, this strikes me as exactly the opposite of toxic masculinity: he's trying to respect your preferences as to how you want to be treated, and in order to do that he has to know what they are first.

(I suppose in the ideal world, there would be enough out trans folks around that cis people would learn how to interact correctly in childhood, but that's not the case yet so we've all gotta learn somewhere.)

Also, you say in a comment that you thought he knew you because you hung out numerous times, but if he just found out you're trans and/or just became comfortable enough to broach the subject to ask these questions, then he clearly didn't know you as well as you think he did.

I had no idea thr opposite was a thing tbh. Personally, I've always treated boys and girls (or men and women) differently, I assumed everyone did this ? Not deliberately -as a matter of fact I've only noticed rather late, probably in my late teens. I can only surmise that it's social conditioning. Just like in your story, it became evident when I met someone who was neither obviously a man nor obviously a woman: I realized I didn't know how to act and was completely thrown off.
Lately I have come to observe that the sexual differences I thought were deep were in fact pretty shallow, and I found myself trying to "see" men as women and conversely, in an effort to break down this conditioning. But to be frank I thought I was doing something completely original... I would never have guessed anyone else had naturally attained this ambitious goal I set for myself !

I mean, I exhibit those same differences in behavior, it's just that it's based on the person and not their gender. Some of my friends and I flirt, knowing we don't actually mean anything by it. Some want to hear my TMI stories and knowledge. Some would be incredibly uncomfortable with that. Some of my friends come to me with problems, wanting solutions, and some just want to talk about the problem. Some of these things are more likely to fall one way or the other based on gender, but everyone's different and treating you in stuff like this based on your gender instead of you is wack.

Definitely not purely a case of toxic masculinity. Gender roles and norms exist in all cultures, even those that were matriarchal.

We can debate all day of that's a good thing or not, but it isn't going away any time soon.

Mind you, the examples they gave are pretty lousy ones, despite being currently a part of general existence. The things they used are part of how women and men expect to be treated, and there's likely a majority that want those behaviors. No telling how many would want them because they genuinely prefer them, or because they've been trained to prefer them over time.

We all know that individuals vary, that there's no 100% possible with what a given person wants in regards to gender related interactions. There's just no way to exclude any of those as valid preferences either. Despite the obvious toxicity of the flirt/chill dynamic where that's the only two options and the default presence of sex needing to be figured out as excluded or not, those things do matter in heterosexual interactions. It isn't solely toxic masculinity and the patriarchal paradigm either, there's an underlying question of "what are our gender norms going to be?" that exists any time there isn't already a rigid cultural standard.

One thing I've noticed with friends that I've known before and after transition is the same confusion present here. It's as hard or harder to learn social expectations that you didn't pick up as you grew up than the physical experiences of transition. Part of that is that we learn a lot by going through puberty and navigating the hormonal shifts at a younger age when we're cis.

People that transition as adults seem to run into greater difficulty absorbing all the things we're expected to just figure out on our own during puberty. Part of learning to be a man is learning how to manage and control the behavioral aspects of testosterone. So many of the unwritten rules of being a man are driven by that need. The same is true for women, but the precise aspects differ to some degree or another (usually to a high degree).

I'm not saying any given behaviors should be a part of overall gender expectations, just saying that they exist and that means an extra layer of things that nobody seems to think of until they're involved in transition (either by going through it, or being close to someone that is).

Being real, I was in my thirties before I figured out that the whole "will we?" question isn't irrelevant, and can't be totally hand waved away intellectually. Doesn't matter what I do or believe, because there's plenty of times it'll arise in other people needing it answered. I had always thought that if you're around a woman or girl that's a friend, you just ignore the sex issue because it'll resolve itself over time and not need addressing. But that's not true. A lot of women need that addressed just so that they can relax around a male friend.

That's not just because men tend to assertiveness and/or aggression in sexual matters. Women can get wrapped up in their drives too, so its usually better to have the possibility of sex and romance (together or separately) worked out early on. It isn't like we don't sometimes want or need sex as a human interaction without it being something more, so even if you're both certain that romance is off the table, sex may not be, and that can come from either party, regardless of gender or orientation (I've been sticking to hetero interactions here because your post is about male/female interactions, but the same need to work out the issues exists with homogender friendship as well).

And none of that gets into the other aspects of gender related minefields that exist for someone transitioning. There's a ton of them that are hard enough to figure out at all, and trying to do so when you've never experienced the complexity of things directly is harder. Just the nuances of how men adjust their behaviors around other men they don't know is ridiculously complex, as one example that I've seen come up frequently. How do you learn all the little body language rules when you've never had to use them in a fluid way? It's as hard as unlearning them when transitioning to female. Having access to hormone treatment doesn't grant automatic understanding of such things.

All of that gets tangential to your ultimate question though. It is true that a shit ton of cis people see the world that way, and even more follow that standard because it is so engrained that you can't just discard it all without running into trouble.

Again, that's not about what should or shouldn't be, it's just an observation about how things are currently.