"[By limiting the affair to one week] we can end it before things get complicated", Sumie said. Of course that was never going to stick, but at this point Manatsu seems properly infatuated. If next chapter this is really escalating to the bed, there's a very high chance Manatsu will chicken out. I assume she sees sex as something one should do with a person they like (especially the first time), and while this condition is probably fulfilled on her end, if Sumie can't reassure her that she feels the same, that might hurt enough that Manatsu will want to pull back.
Maybe I'm overthinking it. Part of me doesn't want this to go too easy, not just because I'm an angst fiend but because I want to see this concept explored to its full potential. I'm not sure if I can really put my finger on it, but the way Manatsu trusts Sumie while knowing so little about her, combined with the looming time limit, squeezes my heart with anxiety.