Kekkon Suru tte Itta Yone?

Growing up, Hiro promised Mana that he would marry her when he turned 18, but when he saw his own father being annoyed by his family, he decided he did not want to marry anymore. However, when he enters his senior year of high school, Mana has become a totally unstable girl who pushes him to get married every day!

(Source: Shueisha, translated)