Umibe no Onnanoko [Manga]

Koume Sato and Kosuke Isobe are two teenagers living in a sleepy seaside town. After getting used and dumped by her crush, the emotionally damaged Koume decides to start a sexual relationship with Kosuke, without any emotions involved. However, they both soon discover that sex with no strings attached do lead to unexpected complications, not just for themselves but also the people surrounding them.