Kanojo no Mama ga H Sugite Gaman Dekinai

When his girlfriend brought him over to her place, what greeted him there was the unladylike figure of her mother, just barely clad in a bath towel and nothing else. The moment her mother left the two of them by themselves in her daughter's room, he immediately pushed his girlfriend down on her bed. However, her mother was also standing by just outside, as she had a good idea of what a couple their age would do. She then firmly lectured the boyfriend for her daughter's sake, but he was already horny and his self-control had reached its limit, so he ended up losing control... This is a story about sex between a boy who's just come of age, brimming with sexual desire, and his girlfriend's mother, who hasn't had sex in a long time and just couldn't handle it. (Source: FANZA, translated)