So who is the cheating one? I see the tag, it should be the wife, right? Then I can't understand why the husband apologizes to her, did he cheat as well?
It's been so long since I last read this but yes, Manabu/husband did cheat. Not sure how far he went before Nanami and Nozumo started fooling around but he was favoring one of the hostess of a bar he frequented. Not sure if you've read this but it was a blonde woman with a kid. The most we see Manabu doing anything with her when Nanami was slowly developing feelings for Nozomu were a blow job and actually sleeping with her (with protection).