Interesting, and yeah I can see it being Kaori, albeit personally I'm thinking it might be Maya.
They almost kissed recently, Sensei has been trying to seduce her ever since, and if they do anything, it'll presumably break him, which implies it's something he's not supposed to have according to "the world" itself/herself (Maya herself fits that perfectly).
There's also how it was Yasu's god who said that:
and afterwards Yasu noted that her god apparently "wants" Maya:
HOPE/Nozomu which seems to be Yasu's god has made it clear it wants to devour those that Sensei loves, and it seems likely that Maya is at the top of the list.
Then again, I could be completely wrong.
Sana could also be a possibility, as could Rin, especially since those two might even be related to Sensei if it was his brother who knocked up their bio moms. Maybe it was even Nodoka? It definitely felt more taboo than usual imo.
Also it's probably a stretch, but the shapeshifter herself also might fit. She claims to not be that kind of character, which fits the not supposed to have part, by design. Then add on the possibility that she's some kind of supernatural daughter of his, and that makes it even more taboo.