um hi?

so far the best explanation that i can come up with is that it's very common for she-cats to have random cats father their kits and their "mate" is just who they love or who they want to play the role of a second caretaker for the kits. sometimes they'll have kits with him but other times the kits in one litter will have a different or even multiple fathers (this is possible with real cats)

so being "mates" with someone is more of a social thing and sexual fidelity isn't that important. if we ignore the cats' "fathers" and assume their real fathers are random rogues (which is plausible because she-cats canonically don't have to reveal who the father of their kits was), the incest is a little less bad and the genetics make a little more sense

the problem with having a mate from a different clan/outside the clan then is that you're emotionally bonded with someone who isn't loyal to your clan. if you just have kits with him and neither of you are emotionally invested it's not a problem (unless you're a medicine cat i guess)

if we only look at matrilineal descent and assume all the genetic material suppliers were random toms, the infamous moonpaw's family tree is just fine, no circles

even if we take the "kin of your kin" prophecy literally and assume dovewing, lionblaze, and jayfeather MUST be genetically related to firestar, it's still fine if we assume the father of cinderheart's kits isn't lionblaze but some random tom and/or moonpaw's father isn't bayshine but some random tom