While I have always really liked Roxy and Calliope together, I have also found that the way the comic sort of just dances around calling their relationship something at all as a bit strange, especially since the Epilogues make it a point to almost obfuscate what their relationship is like and how it works or whatever, as if it's something possibly more strange or different than any other "weird" queer relationship in Homestuck. And I do get what you mean about the descriptors for asexuality being used in conjunction with Calliope's "otherness", like they're almost combining the two in some way that can be a bit offensive. I think a few people here are missing the general point, which isn't that the descriptors are "accurate" to Calliope's appearance, so therefore it's fine, it's more so the context for that quote and the tone it's written in.

Tone is just as important when writing a story, and sometimes biases and bigotries slip through when we're writing. It reminds me of how, in older critiques or writings for the character Wonder Woman, people would associate the Amazons and Lesbianism together. There is nothing wrong with writing about Themyscira being an "island of lesbians" or Themyscira having primarily lesbians on it. But it's the way you describe the island and Wonder Woman that matters.

"Themyscira is an island full of only women, is ruled by women, and has a lot of lesbian relationships." is a lot different from "Wonder Woman is a tall, busty, Amazonian woman from an island of lesbians."

In the same vein, I would think saying "Calliope is Roxy's asexual partner, and they've been living together for years," is different than "Roxy's possibly asexual, alien, skeleton monster girlfriend who she lives with." The first sentence doesn't list a bunch of the descriptors for Calliope and tells you more about her, such as her name, what she's been up to, and even how she might identify. The second is incredibly invalidating and uses those descriptors in a completely non-neutral tone. The descriptors are just there to only tell you about Calliope's appearance and not what is actually important to the sentence and meat (lol) of her character, which is her queerness in relation to Roxy's.

I hope I'm explaining this right, but I couldn't help but read this and agree because authors do this same thing but with lesbians (unfortunately) and I can relate heavily to you OP.