Yeah, I don't take too much issue with it
if it's presented as a choice (as it seems to be implied to now be); but when it was initially mentioned on the forums it wasn't prefaced as though it was to be a choice the PC could make (to have to option to romance / reject the said NPC),
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In that regard (if it is not to be a decision the PC can make), it wouldn't exactly be the same situation as with Brandy; because Brandy only seeks a relationship with the PC if they sex her, and will only see someone else if the PC rejects her. If the PC doesn't sex her and just improves their relationship with her she'll just remain as is.
That did not seem to be the case in the aforementioned CoC2 NPC scenario; as it seemingly implied that the NPC would see someone else regardless of your PC's romantic involvement with them; just due to the fact that your PC had Kinu with Kiyoko - which as others said; sounded like more incentive to not have Kinu in the first place (or at least not free them from the Astral Plane).
Although, I get that the above is just speculation on content not yet in-game; just thought I'd mention it in any case.
So, TLDR - I don't take issue with it if it is presented as a choice the PC can make; however if it isn't, and is something the PC has no dominion over (regardless of their prior standings with said NPC), then I'd rethink the whole freeing from Astral Plane scenario for future play-throughs.