Well, both couples continue their mutual cucking but Chinami seems to truly love Fumiya and only does it with the other guy because Fumiya doesn't touch her. Fumiya is still doing it with Naruse and she is the one who begs him for it while proclaiming her love for him.
Queue 10 years after and both their kids are dating. Naa-kun is a carbon copy of Fumiya (the hair never lies) and Shiki is the spitting face of Naruse. She complains that Naa-kun spends more time with her brother than her, that he seems more like is brother than her, her real sister. He says that he doesn't know why but he thinks of her brother as his own brother and he can't leave him feeling lonely (wink, wink). He then jokes that she is jealous and she says that she inherited it from her parents (wink, wink).
So, Chinami uses the guy for sexual release but still loves Fumiya. Naruse loves Fumiya and proclaims that she feels jealous that he still spends time with his wife. And it is HEAVILY implied that Takahiro is raising Fumiya's kid. Natch.