Yo tellin me, the fathers...uhhhhh...(ahem) son turned into girl, father steals son from his wife......ehhh, I don't this it's it but it looks like it. Ah, oh well.
I skimmed it: some randon virus transformed the father into a woman some years ago. The son falls in love with his father and they fall into a sexual relationship. The father gets drunk at work and his kouhai takes her to a love hotel and takes advantage. The father sobers up while sexing the kouhai and is seduced by the kouhai's sex technique. The kouhai also entices the father to marry.
Eventually, the father increasingly has sexual meetings with the kouhai, lying to the son about where she goes after work. At the end, the father reveals to the son she's getting married to the kouhai: she says it's for the family, but even more importantly, she's in love with the kouhai. The son is devastated and the story ends with no real closure on his part.