[Melon no Hoshiboshi (Hoshiduki Melon)] Kyonyuu Kaa-san ga Boku no Me mo Mae de Oji-san no Namachin de Ikimakutta Hanashi [English] [Fated Circle] [Digital]
How is this NTR? The husband cheated and ran off. The wife is single now, the dude is also single, they both somewhat agreed on continuing the relationship. It is only the son that got off on hearing his mother's moan. The son got a weird fetish but he isn't in any sexual relationship with his mother to justify this being a NTR story.
Ridiculous to just come into the house of your old high school crush and just rape her into loving you. Only hot part is the son being a voyeur to his mom’s sex life and her knowledge of it. Not really ntr either but whatever, the threshold for ntr is so light for normies.