The first couple: Yuuji lives alone. A ghost, Kousuke, suddenly appeared and started to sexually harass him. This happens for a while until Yuuji snaps. Then he bumps into Kousuke but he's alive. Turns out he's some kind of vengefull spirit (is being made by a person with strong emotions towards someone - and he's horny for him. + the living person remembers what he did as a ghost). Kousuke is super horny for Yuuji but stopped vissiting him - cause Yuuji snapped. Anyway Yuuji still thinks about him so he confronts the alive version of Kousuke. They end up as a couple and they can have a "threesome" - Yuuji as uke, KOusuke as 2 semes - he's still concious when his spirit runs around somehow.
Second couple: Yuuji's neighbour (seme) has a friend in the hospital (patient, uke). This patient is the same thing as Kousuke (the vengefull spirit). They end up together.
The extra: bondage fetish - rope (BDSM)
Dos Obake Ga Nekasetekurenai