I think ppl forget that though it is indeed fiction, it's still realistic. It sucks for Ue that Mafuyu is still undeniably dragging around feelings for yuki even though they're together and it sucks that Ue probably feels insecure about the whole thing but Mafuyu's human, he's inherently selfish and self-serving. He can't just forget his first love that easily even if he found someone new. He's still grieving. He's still in the process of moving on. If anything, they should have waited a little before dating.
True that.
Everyone just seems to be expecting the same kind of relationship we're used to in other mangas and just forget that each couple has its own dynamic. They want a normal couple and ignore that Mafuyu has a very extreme background in contradistinction to Uenoyama, who's pretty much a normal teen. It's obvious that Mafuyu will not develop his feelings the same way Uenoyama does but that doesn't mean he's not trying to make the relationship work.
I think people love 'traumatized characters' until they show signs of trauma.
Why are yall being so mean towards Mafuyu? It's not that easy to overcome such a traumatic experience like loosing someone you love. Being realistic, he may never be able to 'forget' Yuuki but that doesn't mean he can't love Uenoyama.