Shirau ga Uchuujin

Emily, a transfer student, arrives at Kenji's high school. Kenji falls in love at first sight with the new student, but Emily is in fact an alien from the planet Chiara, an agent with the role of investigating the Earth in the form of a human mimic. As their civilisation developed, they began to degenerate as a species and eventually lost even the instinct to mate, so they came to Earth to learn about love and sex in order to ensure the survival of their species.  Source: Wikipedia