I want to say this is related to senjou/sumerbreeze but idk where it fits in...
Likely after Kosuzu gave her consent in Senjou.
But yeah the transition from futa to dildo is strange. Usually it's the other way round.
It would be interesting to see their relationship develop and their future prospects, though that is a difficult subject to broach when most human/youkai relationships end with someone being assimilated or passing on.