This is your friendly reminder that shipping Kara/Alex doesn’t mean you’re “erasing” adoptive families. You’re talking about a hypothetical dynamic between two specific fictional characters, one that you understand involves a departure from their canon relationship. You aren’t talking about anyone else’s real family, or invalidating their family. I know that the vast majority of Kara/Alex shippers fully support the canon written dynamic and only want to engage in the romantic shipping in a fanon context, and you don’t deserve to be told that you don’t value or are purposefully sexualizing a sibling relationship. You are not. You are discussing a “what-if” scenario where the “what if” is “what if they saw each other differently?” You are, more than likely, picking up on tropes between the two of them that are heavily romantically coded, and while it’s an interesting thought experiment and a wildly positive thing in the context of mass media to see a relationship like that given the same weight as a romantic one–you are not a creator of mass media, and you are not fundamentally altering or disrespecting the canon text by interpreting it differently. That is one of the core tenets of fandom: The canon stands. And, shockingly, you can hold more than one interpretation of a text, even if the two (or three, or four) interpretations conflict.
Second, “erasure” is a social justice term applied to groups that face systemic societal oppression, and claiming “erasure” is in fact appropriating the term in order to give their personal squick discursive weight where it doesn’t have any. Is it personally irritating? Maybe. Is it possibly something they’ve had to deal with people insinuating about their family at multiple points in their lives, and they just don’t want to hear about it? Sure; and it’s basic decency to respect their wishes to not have people trying to talk to them about things that upset them.
Do adoptive families face systemic oppression that can include things like: Markedly shorter lifespans for people from that group, due to lack of housing, job, or economic prospects (e.g., do people not want to hire them, work with them, house them, etc. because they’re adopted?); entire conservative institutions dedicated to “protecting the family” from them; face potential firing, physical or sexual harassment, or even murder because of their status–so often that it’s necessary to create legal statutes specifying violence based on those characteristics as “hate crimes”; pathologization and attempts to forcibly institutionalize and “treat” them; or ban from engaging in activities or institutions considered benefits, privileges, or duties of “full citizenship” in society, like military service (however you may feel about that), marriage, or (pointedly) adoption?
No.
Are people mean to them for it? Yes. Do people bully them for it? Yes. Do people insinuate things about their families? Yes, and I suspect that’s part of the reason for the reaction some people with that background have. But the necessary implication of “erasure” is that the group in question is also systemically oppressed. And that’s not true in this case.