No one operates under the assumption that the fantasy porn game will third-wheel them at every chance unless told otherwise.
Also: Aileh, Atani, Brienne, Cassia, Cat Harem, Clementhine, Elthara, Eubicha, Gwyneth, Hellhound, Hreatha, Infrith, Livrea, Kiyoko, Ragnild, Rina, Rindo, Serena, Shar, Valencia, Vari, Vatia, Zo.
And that's without counting the more casual ones like Kohaku, Vatia, Hethia ecc, or the bluerrier ones like subArona, Zhara, Kurako, every KM cow, and all them others.
So yeah, Kiyoko and Brienne aren't the exceptions, and it's not really rare at all for people to be solely into PC when we're talking relationships deeper than random encounters (hence every pregnancy character being in that first list). Because that's what people are into. As opposed to getting fucked on by your own companions because "cashaul shex makesh shenshe in-univershe" (but more like writers make a mess of their characters because reasons).