[Toroniku Yukke] Ore no Hatsukoi no Kakure Kyonyuu Moto Coach ga Hoka no Otoko to Kekkon Shite Shimatta kedo Mattaku Shiawasesou ja Nai node Ore ga Netoru Koto ni Shita
She was blackmailed into marrying some guy who found out she was sleeping with her student so she married him to protect MC. This is basically a post-NTR story 6 years later. MC finds out what happened and how unhappy she is and he takes her back and makes her happy. She's cheating on her husband for most of this work, but it's hard to consider it NTR towards him when he only had her through blackmail