so i'm just gonna write this real quick because i am seeing it again and i just want to get my thoughts out:
fans can write whatever the fuck they want, no matter how offensive it is or how "against canon" it is. yes, that includes stuff that "erases" a character's canon identities. can it be gross? yep. am i going to stop anyone from doing it? nope. i will instead block that person and send notes to the fandom buddies i trust to block them as well. i will write or rec the stuff that isn't offensive. fanfic is fanfic and the solution to fans being shitty is to not give the shitty takes any oxygen.
look, i have always shipped fjord and cad as lifelong qpr 'may be occasionally down for sex as stress relief but it and romance otherwise play no meaningful roles in their relationship' partners. i may have changed the tags i use on ao3 for their relationship status (mostly bc i didn't want to get blasted by the gatekeep police but here we are anyway) but my views on their dynamic have not really changed because i have always viewed caduceus in particular through an aroace lens. i ship fjord and caduceus. and i always will because i love their dynamic and i love how their individual themes and struggles can weave together with them as partners. and i like the fact that their relationship is canonically difficult for the two of them to define bc boy howdy do i relate to that. they are family, sure, but they're not brothers, they're not teacher/student, they're not strictly friends bc honestly i don't think caduceus has those strict categories of which love is which. but fjord is special to caduceus and obviously so throughout the campaign. even when caduceus comes to see jester and beau as his sisters, for example, there's a very clear cut dynamic there that there isn't with fjord. their having a very difficult to label relationship and exploring what that looks like and entails, that means something to me bc that's me.
i'm not sure romantic love is even a thing that people feel. you ask me if i'm interested in a sexual and/or romantic relationship, i say, no, i'm interested in a relationship and we go round in circles for three hours as we misunderstand each other bc again, sexual attraction and romantic love do not exist and you cannot convince me they do. the vocab for how i view my relationships, how i want my relationships defined, does not exist.
and here's the thing: there are no meaningful explorations of what relationships outside the romantic/sexual/platonic boundaries society imposes on people with in it and stories produced by it. there is no way to say 'this person is the most important person in my life, i want to build a life centered with them' without those romantic and sexual tags attached to them. so yeah, i will project like a motherfucker onto caduceus and explore it for myself bc finally, i have a character who can see things the way i do and with whom i can process all that shit.
there is no right or wrong way to be ace/aro, there is no erasing sexual or romantic attraction identities by being in a relationship or by writing an aroace character in a relationship. there are better and worse ways to write them, duh, and people are welcome to read whatever the fuck they want and block what they don't.
i guess my other complaint on this is that it's only cad who's being held to this standard. he's not interested in "what you all have going on" in canon. cool. fjord and caleb aren't interested in each other, neither are yasha and jester yet somehow that's okay but some fan writing cad in a relationship isn't? essek's demi-ace but i don't see people saying his relationship with caleb invalidates his asexuality.
in closing once again, ace/aro are sexual and romantic attraction models, actual acts undertaken are completely separate beasts that have nothing to do with the attraction models in question.