What is the ruling on one who says that Islam is a religion of monogamy?

Whoever denies polygyny entirely is disbelieving in the Qur'an and Sunnah (definition), because Allah has permitted polygyny by His saying: {then marry such women as seem good to you, two, three, or four [of your choice]} [Al-Nisa: 3], and it is the practice of the Prophet and his Companions. As for one who affirms the permissibility of polygyny but believes that the default is a single wife, the matter is open to consideration and ijtihad (definition).