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ikr, kinda unusualXD
I think in his mind it was conformed that it was mutual by them having sex and uke being willing and not trying to leave
http://www.mangago.zone/read-manga/sekai_wa_shippo_de_dekiteiru/uu/c0d8fe6ceea47d918ba8fb13f91e21d3_chapter-4/pg-2/
that is why he was confused and angry when uke didn't think it was mutual. his reaction here:
http://www.mangago.zone/read-manga/sekai_wa_shippo_de_dekiteiru/uu/c0d8fe6ceea47d918ba8fb13f91e21d3_chapter-4/pg-30/
I guess that was enough for him, probably because he is a bit more animal like, so he gives more appeases to actions (and smells) then to just word, so he kinda didn't think he had to also say it and confirm it for his human partner ╮( ̄▽ ̄)╭
"No! I don't want only a physical relationship!" *emotional*
"...Oh, you actually love me? Cool." *thinks about what to have for dinner*
(Where was his relief or joy or whatever in response to finding out it was mutual? How blase!)