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I bet Uenoyama and Mafuyu's first time will almost certainly be a little dubcon-ish now (shivering, crying and plenty of "wait!" phrases), based on every other couple we've seen.
But how to turn someone as sweet and considerate as Uenoyama into an asshole on Akihiko/Shizu's level? Simple: just make him incredibly insecure and jealous because he thinks Mafuyu's still in love with Yuki. The scene writes itself.
Yes, it's just a blatant copy of Shizu's arch, but the author has already been copying the same story twice now, so not like she's opposed to the idea.
And yes, I predict this arch too will resolve when someone decides to sing a song on stage.