It's a doujin around 500 pages long, that I read before the site had gone down. If I remember correctly, it starts off with the gf and bf in a room getting ready to fuck and he immediately goes after her breast. The relationship between the two was perfectly depicted as genuine and she adored how much he loved to suck on her tits for her milk. They were both the same age, possible childhood friends, in or about to be out of high school and the mother of the gf also lactated but I don't think they ever did anything with her. The cover showed a blonde girl with large breast and the background may have been pink.