As a non-binary person, I often get asked, upon stating my gender identity, this question: "Are you AMAB or AFAB?", and quite frankly, I hate it, and I think it reeks of bad intentions. Now, I don't think anyone who asks this is explicitly enbyphobic. There's a good chance that they just simply might not understand, but to me, at the end of the day, it reeks of the toxic mentality that assigned sex at birth is a "trait" or a "state of being". I don't see it that way. As someone who is 23 years old, my assigned sex at birth is an event that happened 23 years ago and has no bearing on any inherent part of who I am in the present moment.
This is also why I always speak in past tense for these matters. For example, I never say things like "I'm AMAB", and I feel repulsed in those insane contexts where AMAB and AFAB are used as nouns (like someone saying "I have a question for all the AFABs here" just disgusts me). I always explicitly say, in my context, "I WAS assigned male at birth", and I leave it at that. If you're non-binary yourself and prefer to handle these matters differently, that's okay, as long as you're not projecting that I should go along with your view of this subject. I just like to emphasize that, for me, it's really contingent on context when it comes to how willing I am to state my assigned sex at birth.
The caveat is that, in most of these instances, people are not directly asking me what sex I was assigned at birth. Let me give you a few examples:
- I take hormones, right? If someone wants to ask me about my experiences for whatever good faith reason like relating to it or just wanting more information in order to be supportive of my transition journey, then that's totally fine. With this in mind, people won't ask me things like "Are you AMAB or AFAB?", but instead, will ask me things like "Are you taking estrogen, or are you taking testosterone?" which I find significantly more acceptable.
- If you're my doctor, and you need to know something that pertains to my reproductive system, reproductive capacity, general anatomy/physiology, or any other thing that makes knowing my assigned sex at birth crucial to know, then I don't mind the doctor asking obviously. Like I said, though, often times, a doctor won't ask me things like "Are you AMAB or AFAB?", obviously. They will ask me things like "Do you have a prostate? Can you get pregnant? Can you impregnate others?" and all that stuff that makes the contextual aspect of it all the more sensible.
- For people interested in me in a sexual context who find it hard to assume my assigned sex at birth because of my androgynous gender presentation, asking me "Do you have a penis or a vagina?" makes sense, especially if I indicated reciprocating sexual interest here. In these contexts, knowing my genital configuration would be important to know how to proceed with me in a sexual manner. However, let me clarify that not all non-binary people who were assigned male at birth have penises and not all non-binary people who were assigned female at birth have vaginas.
- The last point I'd state is to make sense of some more niche contexts here. For example, I talk about how short I am all the time, right? I stand at a height of 5'5", which by the standard of an adult cisgender woman, is generally not considered short. If I claim I am short at my height of 5'5", sometimes I like to clarify that, even though I'm non-binary, I was assigned male at birth to make it make sense when I'm on a forum or community for vertically challenged individuals like myself.
At the end of the day, I just like to sniff out context. I hate to be pedantic, but whenever someone asks me things like "AMAB or AFAB?" and leads it on with the fact that they're "just curious", I question the "why" of their curiosity. As I see it, unless the context indicates it being relevant, it shouldn't be taken as an important matter. It seems like a fishy tactic incorporated by those with a bioessentialist mindset to tie my assigned sex as a trait and pick out what my "true gender" is rather than accepting me as non-binary at face value. Like I said to start, many people who ask this are not doing so in bad faith. Often times, they're just misinformed, so I try to explain to them some of the more problematic aspects of said questions being asked in unnecessary, irrelevant contexts.
In a different vein, one thing I wanted to bring up is that language can be a fun tool to get normative people to understand what weird assumptions they are making and what they're doing with pieces of information like your sex assigned at birth.
You already mentioned asking people why they might be asking a question, but I'd suggest slightly more pointed questions... for example:
I also very much enjoy answering questions which fall into yes/no or choosing a category in unexpected ways, when people asking me if I'm a boy or a girl, I like to answer with "I'm a bunny!" because it's unexpected, disarming, and cute in a way that often diffuses and helps people to understand that I'm not interested in answering their question. Sadly almost no one asks me if I have a penis or a vagina, because this is one I get to honestly answer "Yes" to and for it to be completely correct (this is also my answer to the question- what's in your pants?). I spend a lot of time around queer people so I don't get too many probing/weird questions about my sex/gender, but language can be a fun tool if you think a little outside the box of how people expect you to answer and start thinking about how you might get someone to challenge their assumptions.
I'm going to lead with saying I'm cis, and this also likely depends on how someone knows you. The "I'm a bunny" thing could probably easily be taken the wrong way by some people. It's very close to the "I identify as an attack helicopter" transphobic meme. I'm sure it's fine in the context you're using it, but I personally wouldn't use it because it would likely come off poorly and some people may think I am making fun of them.
I am extremely visibly queer, but perhaps more importantly the way a queer person asks about my Identity is not going to be 'what are you?' or 'are you a boy or a girl?'. While I appreciate the concern, this shows a lack of knowledge of queer culture, not to mention the frequent overlap with furry identities and otherkin. There's a plethora of reasons that attack helicopter identification is not an apt analogy here.
In your example though, you said "when people [ask] me if I'm a boy or a girl...", so it's possible the above poster figured you were being asked that by a cis hetero person (since, as you point out, you're unlikely to be asked that by a queer person). In that case, odds go up that the attack helicopter grossness might apply.
Gender diverse people have always associated our genders to things that are unrelated to gender as a way to shown a separation from the binary system.
This is different from queer-antagonistic meme you mentioned.
I would suggest you to read about queer gender dynamics (many of which make reference to animals) and xenogenders.