انیمه Yokorenbo: Immoral Mother

Junko lost her husband about ten years ago and she has been bringing up Ryouichi, her son, alone since then. One day, when he is asleep, she secretly gives him a blowjob and has sex with him. Two years later, she gets remarried.... One night, he accidentally sees Junko and his new father having sex. He remembers what happened two years ago and feels an irresistible impulse to get her back....
(Source: Saishuu.com)