I think the reason so much of the "is headcanoning characters as asexual infantilizing" discourse exists is because of the way fandom loves to associate personality traits with sexuality and sexual preference. Because the simple act of viewing a character as ace is not infantilizing. Not having sex does not make a person - or a character - inherently more childish than a character or person that does.
However, lets not pretend that the characters that often are more broadly headcanoned as ace are not also characters that are broadly viewed by their fandoms as childish, innocent, naive etc. etc. The asexuality isn't what is infantilizing, it is the "omg this character is so sweet and innocent! They *have* to be ace!" whether that is being explicitly said, or present more implicitly in the general conversation surrounding the character.
That is, intentionally or not, making the connection between asexuality and childishness. The implication then being that the character doesn't have sex because they, despite being old enough to definitely know what sex is, somehow do not. And often times, these characters aren't even childish/immature, or even innocent. They are just happier, more optimistic than the other characters that surround them. But even if they were, plenty of immature, childish people have sex. Just look at the average college student, for gods sake.
I would be far less put off by this if the badass grizzled action hero was more commonly headcanoned as ace. (Yes, I know that people do this, I am one of them, but broader fandom in general does not - from what I have noticed at least) Yes, he has been hardened by the world he lives in. He has lived through the horrors. He spends hours brooding in a bar with a glass of whiskey in one hand, a cigarette in the other. And no, he does not want to fuck you, or anyone for that matter, he wants to blast zombies heads off with his big fuck-off gun. But often times this character is portrayed as the sexually confident dom. Why? Because he is more serious and brooding? How does that translate to wanting to be sexually dominant in bed? Answer is that it doesn't.
Personality does not dictate whether or not a person has sex or not, or the way in which they want to have sex. So when we assign certain personality traits to specific sexualities - often unconsciously - we create false narratives, like that all aces are sweet and innocent and would never hurt a fly. Which is. Not how that works. And I don't even mean this in the "ace people actually make the most raunchy jokes, write the best smut etc." way (which is just stereotyping in the opposite direction). I mean it in the, asexual adults are still adults. And them not having sex is simply because they don't experience sexual attraction to some degree. That's it, that is the only requirement. So it becomes deeply weird when the majority of characters that are broadly viewed as ace by the fandom, are also the same characters that are viewed as "childish."
(I could also get in to how adults liking more "childish" things does not equate to being actually *like* a child, and how hobbies also cannot dictate sexuality, but that is another conversation that I do not want to get into right now.)
And of course this isn't to say no one should headcanon a character that is more "innocent/childish" (however that is being defined) as ace, or the serious, world weary badass as being a dom. It's fandom, people can do whatever the fuck they want at the end of the day. This is more about recognizing patterns, and learning to question those patterns.
Just. Be curious! Question why you do things! And hey, the end result of that questioning isn't always going to be "I held unconscious biases to asexual people" It could just as easily be "I relate to this type of character and am myself ace, meaning that I find myself wanting to give those characters traits that I have, one of them being my asexuality."
