Yeah, it’s not ace headcanons that are the problem, it’s the misappropriation and hijacking of asexuality to shit on marginalised people, which is not only racist/ableist etc. but also pretty shitty to ace people who deserve better than having their orientation used for an agenda like that. (Although of course white ace people – like white non-ace LGBT people – are also perfectly capable of perpetuating racism and shitting on/misappropriating the experiences of ace and/or LGBT POC). Although Sherlock is an interesting case because Sherlock Holmes is a character who’s always been considered asexual-coded or interpreted as asexual by scholars (that and gay, because a lot of the things that could be considered evidence of him being aro/ace, like an apparent lack of sexual interest in women and opposition to marriage, can also be considered evidence of him being gay. Or gay and ace, because that is a thing). And I seem to recall that in the BBC show he said something about girlfriends “not being his thing” (or something to that effect) and is/was canonically a virgin, so that’s a pretty good foundation for an asexual reading of the character. The effect is more apparent with characters like Sam Wilson and Mako Mori, who were widely headcanoned as ace after their movies came out despite there being absolutely nothing in canon to suggest that they were asexual beyond just…not having love interests in canon (and being a little too close to the pretty white boy that fandom would prefer to ship with another white dude). (Not that ace people have to always justify their headcanons with canon evidence – there doesn’t have to be a “reason” to headcanon a character as a particular identity – but it was a little too coincidental that all these white non-aspec people were suddenly all aboard the ace!Sam/ace!Mako train). Although sometimes white characters with love interests who are POC (e.g. Rey) also get “headcanoned” as aro/ace to get out of shipping them with their canon boy/girlfriend or justify hating their relationship.