Isn't it only natural for Caliborn and Caliope to be attracted to each other though? Isn't that whole thing the only reason Cherubs search for mates?

It’s a bit more complicated than that. As Aranea says:

And though it was to her 8enefit and personal growth, 8ecause of this loss she would always live with a sense that something was missing. Every sexually mature cheru8 lives with this feeling. It drives them to seek out another cheru8 similar to the half they lost, the part of their 8eing which they grew up in perpetual conflict with. The desire to travel the universe in hopes of reigniting that conflict is very important to their species. It’s the force which compels them to procre8.

Do remember that Cherub Mating is exclusively Hate-Based. Rather than a physical attraction, what they covet is the struggle they went through to predominate, an equivalent to fill in the part they pretty much absorbed. And given the Cherubs are coded to be Good or Evil, this works to their advantage since half of their entire population is going to be able to fill in that place.

Now, however, this is normal Cherubs, Cherubs who grow up in complete and utter Isolation, who may not see or interact without another being for their entire lives except when the time comes to mate. Calliope and Caliborn break this mold, as they interact with Humans and Trolls and have their perception of the world changed by them, they take upon Social Constructs and move beyond normal Cherub Behavior.

This shines through Calliope in the way she learns that, hey, yeah she can definitely feel Flushed and Pale for others, and her Feelings are not bound to Hatred, and she writes all sorts of embarrassing, sappy, and potentially even smutty tales of her OC and her Friends. Conversely, Caliborn, rather than learning love and affection as natural things, fetishizes flushed physical attraction and romance, to the point where he the more tender and sweet something is the more he likes it, in the most disturbing of ways.

So Caliborn doesn’t, at any point, shy away from hating Calliope. She has her always as the ultimate thing that must be destroyed, how beautiful it is to kill her, painting his words with her blood, that’s. Awful but apparently more akin to normal Cherub Behavior. The fact he sees her as a Sister, however, already taints the normal Cherub conventions of her merely being another self he has to predominate over, and further likening her to figures he shows a flushed attraction towards, indeed, cements this as not just ‘Normal Cherub Behavior’, and much closer to ‘The Concept of Human Incest’.

And at the same time, something like that ALSO cements the idea of Caliborn as a Narcissist, stroking his own ego, given that, physically, except for their eye color, they share the exact same body. Forever, too, or at least until he becomes Lord English, since he stumped his growth by predominating early, he’ll forever look like a red-eyed version of Calliope, except for his. You know, robot leg and gold tooth.