How much of this sexuality discourse do you think is rooted in thinking one sexuality is superior to another (or, conversely, thinking liking men is worse than liking women)?
also i'm so sorry for all the asks you've been getting recently for having correct takes
i think less of it is about pure superiority and more about being unable to relate to experiences that aren't your own.
the trend you'll see with a lot of queer headcanons, especially ones that go explicitly against canon, is "well they're my favorite and i'm a lesbian so they have to be a lesbian". which is obviously fine in a very "playing dolls and not actually analyzing the text" way, you're allowed to have fun, but people lose the plot a little [quite literally lmfao] when they start implying that those headcanons are genuinely the case in the actual text of the show. it truly begs the question: why can you ONLY care about this character when they share a sexuality with you? and, if that's the criteria for you, why are you choosing to push that label on a canonically straight character when there ARE real lesbian characters out there? of course, there can always be more queer rep in media, but also some people really aren't trying very hard to engage with what does exist.
i do think there is something to be said about the sort of faux feminism people often weaponize when they call a character a lesbian. "fuck all men, only date women, cheat with a woman and divorce your husband its better for you" etc etc. though it may not look like it at first, this is misogyny: there is nothing inherently "pure" or "better" about sapphic relationships, they can be equally as toxic and devastating as any het relationship; there is a lot of misogyny in ignoring a woman's choices about her own relationships, it doesn't really matter if he's "right for her", it matters if she wants to be with him or not and that applies to wlw relationships as well; treating a grown ass woman as someone who's "lost" and "needs to be saved" is patronizing and misogynistic; equally, the woman placed in the position of the "savior" is being assigned a lot of emotional labor they likely wouldn't want or be able to provide. in that sense, i guess you could say people are painting lesbian relationships as falsely "superior" in some way.
the irony of it all is that i think this entire ideology is actually most harmful to lesbians. telling women that dating other women should "fix" them and implying that those relationships can't be toxic or abusive is actually extremely dangerous rhetoric, especially for trans lesbians who are most often the targets of abuse. i also feel like it plays into this sort of... sanitization? of lesbianism that i've seen especially online, which is just a whole other topic lmfaoo.
i'll also note that a lot of the women people want to hc as lesbians are conveniently the second half of a ship that "interferes" with an m/m ship. in house fandom cuddy is probably the best example of this: do people really care about seeing her as a lesbian, or do they just want to get her out of the way of their preferred yaoi? a lot of fics under f/f on ao3 are also tagged with h/lson, making it pretty obvious what the nature of the femslash ship is in the fic. this is more obvious misogyny.
in general, i have noticed that fandom has a tendency to... commodify queerness, i think is the word. every character gets a queer label whether or not it makes sense in canon, and this is apparently Woke and Queer Activism; i find it interesting that this isn't ever done on a wide scale with other identities. people also like to use labels, transness especially, as a sort of way to ragebait a creator or actor, or even just like... "fix" an otherwise problematic media. people only headcanon chase as trans because jesse spencer is transphobic. it really makes me mad, because you're not at all exploring transness in a genuine way or how that character interacts with gender identity, you're using it as some kind of weird self-satisfying revenge that the person isn't even going to be affected by. that is literally just transphobia. [meanwhile, canon trans characters get ignored or vitriolically hated. interesting isn't it.]
people in fandom pride themselves on their ability to queer-ify everything, even if that queer-ification is actively harmful in many ways, while turning the other way when it comes to misogyny, racism, ableism, etc., a lot of which they are perpetuating through those queer headcanons. some of it is, of course, projection which circles back to my original point: why can you only care about characters you have something in common with? why do you feel the need to disguise that fact and insist something is true in the text when it's not? why are you even engaging with a text that you seemingly want to change in every conceivable way?