How Did That Happen?

Have you ever had sex? As soon as he becomes an adult, Sung-Hyun spends the night with his childhood friend Ji-Won at his request. Sung-Hyun, who has been secretly in love with Ji-Won, tries to make things work out with him, but Ji-Won sleeps with other people as if nothing happened. Then one day, he discovers a rough mark on Ji-Won's body... (Source: Mr.Blue, translated)